One of those things that makes me think someone at Rankin-Bass was actually doing their research, because:
Rounded collar on the man at left, both boys (white coat in front and pink shirt in back right) are wearing knickerbockers.
Santa, dressed but not in his suit, has pinstriped pants, spats, and a watch chain. We can assume he probably dresses a bit older than those around him because he's elderly and doesn't necessarily care about trends, and--what? What's this? A fashion plate from the Edwardian era?
No spats on this guy, but they were common at the time--it might be because spats were practical and the drawing is intended to be fashionable.
Now let's look at some of the parents. Once again, we can expect both that they're going to be dressed ever so slightly older, and also decades don't have hard start/stop lines in fashion and culture like they do on a calendar.
The dove-breasted dress front isn't present in AYEASC, but the length and collar are perfect compared to this photo from 1910, and if we assume the mom is wearing something just a couple of years out of date, the bodice shape lines up very neatly with the end of WWI.
Likewise, look at her hair and how loose it is around her face in spite of being in a bun, and then check out this photo of a Gibson girl:
And then we've got the dad, with his hair parted straight down the middle, his large shirt cuffs, and vest. Check it out:
Also please note Dad is in plaid and as you may see in this ad, that was A Thing in the time period. He's not the only one, either--several of the kids wear it.
Other than Santa's outdated Edwardian attire (which makes sense for his character as an old man), all of these styles date within five years of 1920. Most date to ever so slightly before. These are middle-class people, so we'd expect them to not look like the latest fashion plates. Not only does their clothing make sense for a 1920 date, it actually makes sense for real people of their socioeconomic class in 1920.
Like damn, I know Rankin-Bass looks kind of hokey these days because stop-motion has evolved a lot and their teeth decisions were...interesting, but fuck, I think they may deserve more respect than we're accustomed to giving them.
