New Year's what?

I have often noticed that Americans say "New Years" - and wondered why it was plural.

But just reading Obama's biography I've noticed for the first time it is spelled with an apostrophe.

School started two weeks before Inauguration Day, so after New Year's we flew back to Chicago...

In Britain we would just say "after New Year", or we might say "after New Year's Day". But I am puzzled as to how America got to "New Year's". It could mean a whole lot of things - "New Year's Eve", "New Year's Day", New Year's holiday" (which we would call "the New Year public/bank-holiday*"), "New Year's picnic" etc.

Does anyone have any thoughts about this?

*If New Year's Day falls at the weekend, the Monday following is a public holiday - (often still called a "bank holiday" in Britain).

It appears to be an AmE usage and it may refer to either New Years’s Day or Eve, according to context.

New Year's (uncountable noun)

New Year's is another name for New Year's Day or , New Year's Eve. [US]

(Collins Dictionary)

but according to Grammarly.com:

“New Year’s” usually means “New Year’s Eve,” and people usually specify “New Year’s Day” when they’re talking about January 1.

while according to M-W

New year:

usually New Year's : New Year’s day

thesaurus.com about the usage of “New Year’s” points out that:

you should also use the ‘s even when New Year’s stands alone as long as you’re talking about the holiday. For example: “Let’s plan to get together for New Year’s.”

Here, the ‘s implies the eve or day. You should, however, probably get a bit more specific with your friends so they don’t show up on New Year’s Eve when you’re in your pajamas and were actually inviting them to brunch on New Year’s Day. Just saying.