what year did mid century modern become popular again  

Art Deco, then MCM, now Beaux Arts Revival?
In SoCal, the MCM trend seems to have been prefaced by interest in architectural preservation of certain modernist architecture in SoCal. I believe some of this started in the late 80s or early 90s. I remember a lot of talk about not letting the Shindler and Neutra buildings in LA be wiped out after some were. But that was mostly talk. Serious preservation attemps probably did not get underway in SoCal until the mid or late 90s though I was out of region for awhile during those years and so may not be the most reliable of recollectors.
Regardless, there's nothing like a little boomlet of preservation interest in architecture, to trigger the need for decor to fit it. And LA is a great place to promote such boomlets, because there is always a bit of architecture of one era or another that has fallen into disrepair and can be rehabbed profitably if it can just be rethought of as historically significant and potentially stylish. In turn, Hollywood movies are always looking for the next new thing to give their movies a new, now and nutty look. And it is never long after a rehab revival occurs in LA that some film comes along and uses the rehabbed buildings for sets for some kind of film that spreads "the look" like spore around the country and around the world. Often times the spore infects no one. But sometimes the spore spreads, as it did with art deco, and suddenly what a few collectors and a few architectural buffs thought ought to be presevered, suddenly gets caught up in a ground swell of interest. And the early collectors and architectural buffs feel so vindicated...until the next rehab driven revival starts.
The other vector of interest seems to have originated among collectors, as I have already alluded. The interest in MCM among collectors was a natural progression from a preceding spike in interest in all things art deco in the 80s. Collectors seem to have moved onto the next big thing after art deco, when the easy pickings of collecting art deco, and the films tapped the nostalgia in people, and the advertising agencies had fully exploited that feeling and it then receded. MCM revival came essentially on the heels of art deco revival if memory serves me correctly.
The interesting thing now is that there seems to be the first inklings of a Beaux Arts revival underway, which means to me that the collector population cohort is not quite yet of the age to want to collect its own post modernist nostalgia for the past yet, which, chronologically speaking, would be the next logical revival after the current MCM revival plays out; that post modern revival should come sometime in the next 10-15 years, though, likely after this Beaux Arts boomlet.

pt. 2
The Beaux Art boomlet may be spiked in LA by the rehabilitation of LA's theater district and some Beaux Arts theaters there. To some extent the Art Deco revival of the 80s/90s started with real estate rehabbing of Art Deco theatres etc. These movements seem either triggered by, or to shortly follow spikes in interest in preservation of these types of real estate by historic preservation organizations. Who knows, maybe that follows a spike in collector interest, or drives it? I can only say that there seems to be some dynamic interplay at work among these activities. Most likely there is some chicken and egg at work.
If you stop and think about it, a Beaux Arts preservation boomlet makes some sense. We are living through a second gilded age, or robber baron age 2.0 that is like nothing so much as the era between 1865 and 1900, an era when the Beaux Arts was important and the Functionalist movement was in ascendance--both movements making use of opulent ornamentation, however different their underlying philosophies for using it may have been.
And as I have noted recently in another post, there is a book out now by the former President of Boston College that basically takes modernism and post modernism to task using essentially a Beaux Arts descended philosophy for a framework of criticism. If that is not a mass media indication of a beaux arts boomlet being ushered in, I don't know what is!
Being in one of those historical periods of bottomless corruption and epic wealth asymmetry, as we are today, is it any surprise that the aesthetic attenae of the culture workers (the small and large gate keepers of taste in creation, productions, marketing and collection) would react to it with both MCM astringency, i.e., we must discipline ourselves with elegant, but anal retentive modernist revival on the one hand, and increasingly now with the ascendant retro-baroque, or perhaps retro-decadence of a Beaux Arts revival?

pt. 3
When society and culture are confronted with the rise of decadence, corruption and barbarianism in massively escalating concentrations of power and abuses of that power in government and the economy, when war is contrived to steal oil, when torture of innocents is institutionalized as an asymmetric warfare tactic of a new world order, and when habeas corpus is assaulted for close to a decade after having been incrementally on the ascent since 1250, it only makes sense that after trying the astringency cure of modernist revivalism, society would develop an appetitite for baroque, often meaningless ornamentation exhalting, or at least giving expression to humanity's simultaneous awe and revulsion at what elites and their agents are capable of when let off the leash of enlightened, sovereign, and constitutional government.
Here is how Wikipedia defines Beaux Arts, for better or worse:
"Symmetry.
Hierarchy of spaces, from "noble spaces"?grand entrances and staircases? to utilitarian ones
More or less explicit references to a synthesis of historicist styles and a tendency to eclecticism. An architect was expected to work fluently in a number of "manners", following the requirements of the client and the architectural program.
Precision in design and execution of a profusion of architectural details: balustrades, pilasters, panels of bas-relief, figure sculpture, garlands, cartouches, with a prominent display of richly detailed clasps (agrafes) brackets and supporting consoles.
Subtle use of polychromy."
Functionalist like Louis Sullivan came along and tried to make some sense of this mish mash; then got eclipsed by the early modernists.
It seems the nostalgists wish to re-explore this period before gettting on with a post modernist revival, even in the midst of a heavy modernist revival that is really just post modernism shorn of ornament itself.
Oh, what a tangled web we weave, when first we practice to nostalgically conceive.

Here's my guess
I see that DC continues to provide the 3-volume novel format, but here's my best guess:
Art Deco and Streamline was all the rage in the early 1970's. It was short-lived because there was a nifty, pioneering article in the magazine "NEW WEST" with a cover story "Recycling The '50's" from the January 31, 1977 issue.
I guess that was a good starting point in the 'rage' of the beginnings of collecting 1950's boomerang and flamingos kitsch, as well as iconic Ball Clocks and the beginning of interest in Herman Miller and Knoll stuff (remember, most of these classics were no longer available and there was lots of stuff being discarded.)
The article featured San Francisco junk shoppers who were finding and restoring whatever they found that they consider good design. (On the of the featured pictures was Robert Laughlin...shown below)
On last bit of perspective trivia; in about 1983 or 1984, I was with a friend junking the shops along the Upper East Side in NYC - there was a big crowded second hand store on 3rd Avenue and about 74th Street. My friend found a Nelson Desk in the basement...nearly completely covered in stuff! Imagine finding a Nelson Desk nowadays....

I think you are all corr...
I think you are all correct.
I started collecting kitsch stuff , mostly the funky tv lamps and googie lamps and then the whimsical stuff ,
in the early 80s and we called it then Retro and really never looked at the furniture till the 90s because my home was done with Baker traditional and Micheal Taylor furniture , who was doing a lot of Chinese modern and was super popular back then, he was the go to guy for the great collections he did for Baker, and was truly a hidden talent and his furniture is now going through the roof with Mastercraft you can ask Evan Lobel as that is what he spends a lot of time aquiring that kinda furniture,
I started my modern art collection in 1972 when I moved to Houston and every thing was very modern at that period coming out of the space age and Houston being the Hub, I can remember going to office after office and seeing nothing but mies,Breuer
and warren planter and of course loads of Milo Baughman, for Theyer Coggin,
Karl Springer was the high decorator at the time as was Hollis Jones, for your Lucite needs and skinned coffee tables, and side tables,
Harvey Probber, Kagan, Parzinger,Evans and the pace collection were doing great with the rich,as their stuff back then was somewhat pricey and was in every modern home in Houston in the 70's .
I guess to say the word "mid century modern " was more a evolving word, as all of these guys were around in the middle part of the 20 th century,
Of course Eames,Saarine and others were popular, but i think by the late 60's and 70's they had not kept pace with the other designers as Sarrine died in 1961 and that was it for his furniture design, and Florance Knoll left the company in the mid 60's
I was just thinking if there was a coined period
like 1969 when Bevis Hiller coined that word Art Deco
that en composed streamline modern, Jazz modern and zig zag
I find this all interesting to me as I was very much a live and living in the moment, and I can honestly say i never heard the word midcentury modern design,
side note to Mr. Wilson why do you feel Baux Arts,is the next big wave, any one who whent through architecual school knows this term, but i doubt if you polled 10 people they would
know anything about this period unless you have traveled extensively through parts of NYC and the great cities of Europe, curious why you think this will be the next rave cause the furniture was far from modern and not that great as far as I am concerned , and if it was it could be classified Hollywood Regency sorta.

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Cara Greenberg at least polpularized the term with her book MID CENTURAY MODERN.I was collecting "50s designer modern" pre 1976 when I was in high school and recall talking about 60s design with my art teachers in the 1970s!Retro is a term that is often used for the kitchy stuff.The best stuff gets saved from every era,the problem its near impossible to judge your own period...

Cara Greenberg does get a ...
Cara Greenberg does get a lot credit for the term Mid Century Modern as her book came out in 1984 before that I never heard that term, just the term Retro furniture, all the kitch stuff was also called Retro like Azurechicken said,

LRF...
I am not fond of much Beaux Arts architecture, or design, but collectors and revivalists must always have something to collect and revive.
Art deco collection peaked and receded. MCM seems to be peaking and may perhaps be entering its eclipse phase.
Collectors must have something out of vogue to become enchanted with for that is where margins for collecting originate.
Revivalists must have sufficient artifacts and buildings to build a revival movement on.
Both require a potential market of persons who could be made to thirst for collecting and reviving by way of education by media and market mechanisms.
Persons thirst for collection and revival for either or both of two reasons.
1. They want to collect and revive what they were familiar with and fond of from their childhood; i.e., what they often grow nostalgic for as they age and reach a life phase where they have the growing awareness that the world is no longer as it was when they were young, and have the disposable income to vote with their money to collect and revive a design era of their youth in order that it may be recovered, if only partially.
2. They want to collect and revive what seems antidote to, or harmonic with the dominant political, economic and cultural condition of their times.
In answer to your question why do I think Beaux Arts might experience a brief revival now, I will try to clarify now what I thought out loud about above in my 3 volume set in perhaps yet another 3 volume set. 🙂

pt. 2
First, we're do for a postmodern revival because it is the next most recent and substantial movement to follow MCM.
Second, the age group that would be prone to look nostaligically at postmodernism is not quite old enough to develop a critical mass of collectors and revivalists. No doubt there are young collectors out there right now churning through thrift stores and garage sales looking for jewels of post modernism, as there were young collectors of MCM 15-20 years ago starting the trend under the radar screen. But it takes a while before most of a generation cohort enters the ripest part of the collector years of their lives to create a critical mass for a collector and revival movement and I suspect we are not there yet for postmodernist collectibles.
Third, in the interim while we wait for a spike in postmodern collection and revival, something will inevitably fill the vaccuum.
Fourth, that something may be a spike in Beaux Arts collection and revival, because I think it fundamentally expresses with excess of color and slightly grandiose ornamentation, some what hollow references to classicism, and somewhat elitist symbolism, a lauding of the aristocratic order in unbridled ascendance from their imperial action in the world that I, at least, observe presently in the world.
Fifth, the first Beaux Arts arose largely IMHO to please and appeal to the sensibilities of an age dominated by the robber barons of the first great push by the European-American elites towards globalization of their ruling interests, i.e., globalization version 1.0 in the second half of the 19th century.

pt. 3
Sixth, while Beaux Arts abounds in western Europe, it is less prevalent in USA and so to spike in interest here it will have to be lumped in with the ornamented Functionalist artifacts and architecture that followed and tried to rationalize the Beaux Arts and which left somewhat greater imprint in USAs twilight of the robber baron age. Aristocracies historically, it seems to me, value grandeur and grandiosity as expressions of their sensibilities in periods when their class is having its way pretty much unbridled. People living and suffering under the shadow of aristocratic ascendancies are in particularly poor position to demand (or pay for the consumption of) an art, architecture and artifaction (yep I just coined a word) indicative of the peoples beneath the aristocracy. So the exhuberance of the aristocracy combines with the lower classes' awe at just how wretchedly the aristocracy can wield its excess and so an era of Beaux Arts type ornamental excess manifests that tries to tie itself seriously to some grandiose classicism as well. This is my hypothesis anyway. (Note: post modernism was kind of a half step to a Beaux Arts revival, because it only tied itself tongue in cheek to classicism.) We have such a situation right now it seems to me. The aristocracy is on the rise, wildly indulgent, and taking itself utterly seriously in its attempts to reverse two centuries or so of constitutional republican progress. And so my mind expects the possibility of such a Beaux Arts style spike in collector and revival activities to harmonize with that.
Seventh, I note such a revival of Beaux Arts revival style theaters in Los Angeles already. Do you appreciate the hollowness of this? The theaters being revived in LA are themselves Beaux Arts revivals from the 20s and 30s, if I understand them correctly. So: Los Angelenos have revived a revival. You have to love the naive audacity of it.
Eighth, I also note the arrival on the popular book store shelves of a book blasting modern and post modern architecture and using Beaux Arts philosophy as a yard stick of criticism.
I know none of the above is conclusive. It is just food for thought to watch and see what does unfold.
I apologize to all for the lengthyness, but this is how I think and express new ideas (at least to me) for better or worse.
If I could do it in a few lines, I would.