expressions - Why do some people say "Happy New Years" with an "s" at the end of "years"?

When addressing New Year's Eve and New Year's Day collectively, while simultaneously disambiguating from the date transition, New Year, I will often truncate at just New Year's to indicate those days possessed by the holiday. This seems to be one of the most common understandings of the set of days bracketing the date transition, the other being the use of the capitalized New Year to imply either or both of these cases.

I believe dropping the possessive is a common corruption of the former, due to a basic misunderstanding of how it interacts with the modifiee, "Day," instead of a deliberate appeal to the set of all possible New Years celebrated in a singular, or set of surrounding, Day(s).

This has been discussed previously in other answers and comments, and it'd certainly be an interesting cognitive study in language drift to see why each form is in common use.

Further sourcing and discussion, for British and American English:

http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv329.shtml

http://data.grammarbook.com/blog/apostrophes/confusing-possessives

http://www.englishforums.com/English/AllTheBestForNewYears/kzrhz/post.htm