New Year is the oldest of all holidays, as it was first observed in ancient Babylon as many as 4000 years ago.
Celebrating New Year on January 1 is purely arbitrary, as neither it has agricultural significance nor astronomical. Many countries still celebrate it in spring, the season of rebirth of new crops.
The Roman senate declared January 1 as the New Year in 153 BC. Though even this date saw major tampering, it was Julius Caesar who again declared January 1 in Julian calendar as the New Year, in 46 BC.
New Years is still observed as the Feast of Christ's Circumcision by some denominations. In various South American countries like Bolivia, Ecuador or Brazil, there is this tradition that has to due with yellow underwear. This means that you have to wear it immediately after midnight as it seems that yellow brings good luck. All women in Mexico who want to find their love in the new year have to wear red underwear.