Audubon's Chrismas Bird Count is a wonderful tradition that started in 1900. It's a great event that welcomes birders of all skill levels. These counts no longer are held on Christmas day, but on the days surounding the holiday.
In New York City, the dates are on the weekend before and after Christmas. I'll be attending the Central Park count on Sunday, December 20th.
For more details on the New York City counts, see the NYC Audubon website.