Seems quite silly when you think of it, doesn't it? - a day/night like any other, just the year number changes. Well, I could ask this also about the change of the millenia 4 (or 5, depens on how you like it) years ago.
Do people consider numbers to be somewhat magical?
Or is it just the sense of recognizing that yet another year has passed. And then reflecting on the past year - the good, the bad and the ugly. Or looking into the future - what might become of the next year, what should/could be done differently when compared to the passed year. But, if it is the time for reflection and meditation then why all the parties?
Ah well, I guess it is all about the fact that the number of different attitudes about anything at least equals (but most probably exceeds) the number of people who are having those attitudes.
But in the end, all these parties seem rather pointless, don't they? :-P