For anyone who was not an adult during the 1950s, the era remains shrouded in mystery. After the rising crime rates of the 1960s (which began in 1959), an effort was made to quarantine the earlier stretch of time. If everything seemed to be going wrong in the '60s, then it must have all been going right in the '50s. In particular, the attitude of questioning tradition must have been preceded by a zeitgeist of upholding tradition. Once you peek behind the quarantine curtain, though, you find a culture that not only was not very traditional, but that thought of itself consciously as progressing forward beyond outmoded customs of a pre-industrial age.
There was that whole grand vision of "bringing you the world of tomorrow, today" that proved immensely popular at the 1939 World's Fair, but whose Space Age appeal had already begun to seem naive by the time of The Jetsons in the early '60s ("Jane! Stop this crazy thing!"). There was a new reluctance among parents to beat their kids' ass, suddenly seen as barbaric, and a shift toward the more coddling approach preached by Dr. Spock -- and whose consequences were the bratty bitch daughter in Mildred Pierce and the directionless son in Rebel Without a Cause. The rise of the drive-in for eating out, watching movies, and going to church began to replace the backward peasant habit of sharing these experiences next to other people in public spaces. In place of the earlier tribalistic separation of races, the barriers were to be knocked clean over, first through piecemeal court cases and then culminating in Brown v. Board of Education.
And the mid-century's reigning political and economic beliefs could not have placed greater faith in the ability of experts to surgically tailor public policy for the optimal benefit of all, whether under the supervision of liberal Republicans like Eisenhower or Mr. New Deal himself, FDR. The heyday of faith in state-of-the-art social science had superseded a Medieval approach of puttering around in the darkness of the cloister, groping blindly for solutions.
Two of these currents -- the flippant attitude toward preserving traditions, and social engineering by experts -- came together in the late '30s and early '40s, when the President de-sacralized the Thanksgiving holiday by altering the convention for when it was to be celebrated. Since Lincoln's presidency, it had always been observed on the last Thursday in November, but Roosevelt wanted it moved to the second-to-last Thursday, giving retailers a whole extra week to peddle their wares before Christmas, thereby Doing Something to Help Stimulate the Economy. Within a few years, more or less the whole country was on board with changing the convention, albeit striking a compromise in the form of making it the fourth Thursday instead of the last or next-to-last one.
Why harp over something as inconsequential as which day we celebrate Thanksgiving on? Because the day for a group to celebrate some occasion is a solution to a coordination problem, like which side of the street to drive on. Of course it doesn't matter whether we drive on the left or right side -- as long as everyone is clear on which convention it is. We know deep down that Thanksgiving isn't made more or less special by celebrating it on this, that, or the other day.
But the convention had long been established. We invest these arbitrary solutions to coordination problems with an air of purity and sanctity so that people don't switch among conventions whenever they please, throwing the community into confusion and causing them to lose faith that there is a common convention being adhered to by all the others in one's group.
By violating the established convention, what did Americans gain? Well, nothing for almost everybody -- again, there's no inherent advantage to one day or the other. But a handful of retailers might (or might not) have squeezed a few more pennies out of its consumer base, so it's all good. When two solutions are equally good days for celebrating Thanksgiving, moving from an established day to some other day is a pure loss -- the celebrants are no better off than before, and they had to pay all those costs during the chaotic transition from one norm to another.
Concretely, in 1939 Roosevelt announced the new convention in late October, fucking up all kinds of plans that individuals and organizations had already settled on for the November holiday, including the schedule of Thanksgiving sports matches. Yep, even the sanctity of American college football had to be sacrificed in the service of ushering in more rational policies. Not to mention the loss in communal feeling and action that comes from people believing they no longer belong to as stable of a community as they had believed earlier -- if they're going to mess around with a national holiday willy-nilly, who's to say they won't target other important norms too?
Also bear in mind that this flagrant and nakedly materialist coup was carried off when 90% of the population was white (compared to 72% today), and even where ethnic differences existed among whites (for example speaking Italian vs. speaking German), they had no differing traditions for when to celebrate Thanksgiving. Having a more ethnically mixed-up country makes it even harder to enjoy shared, stable norms, but homogeneity is not so powerful of a benefit as many conservatives believe. I've already covered plenty for mid-century America, but go even further into contemporary Scandinavia, where people have been on a kick for some time now to shift from one set of gender roles to another. Homogeneity helps, but is not a bulwark against topsy-turviness in the realm of social conventions.