If we're mentioning holidays that we made up, here's mine. March 5 (YYYY-3-5, when you write the date big-endian) is the Square Root of Christmas (35 * 35 = 1225). This allows 10 days between Sqrt(Xmas) and Pi Day. This period of time is called Nerdigras.

http://twopoint718.blogspot.com/2009/03/march-5-is-square-ro...

There's already been a carol written:

http://nedbatchelder.com/blog/200903/square_root_of_christma...