Agnostica is an atheist holiday proposed to replace the more traditional Christmas/ Hanukkah/Kwanzaa/ Winter Solstice/ Saturnalia celebrations. These celebrations all emphasize the religious and spiritual to some extent, so Agnostica was designed to side-step these unfortunate traditions, allowing hard-headed atheists to party without reference to anything other than an almost arbitrary date.
Agnostica is celebrated on December 14th, which also happens to be Quantum Mechanics Day, the Bangladeshi Day of the Martyred Intellectuals, Monkey Day, and Tycho Brahe's birthday, none of which are, as far as we know, the reason for the season.
Agnostica apparently appeared first in the webcomic Nukees, in a series that started on December 13, 1999, and culminated with the declaration of Agnostica as the secular holiday of preference, over the illogical and irrational Christmas on December 27, 1999.
Being a new holiday with essentially no tradition, Atheists may celebrate the day in whatever manner they see fit, although there seems to be an emerging trend of celebrating with a group of like-minded friends and drinking beer. The artist behind Nukees, Darren Bleuel, has since suggested traditions that build on Agnostica's shared date with Quantum Mechanics Day, including displaying boxes with unknown contents which remain unopened and the random bag of fun, and some random bits of party fun such as naked jello wrestling, Tang-nog, decorations such as Möbius chains, and costume parties. YMMV.
The 'official' Agnostica website is rather cynical about the Holiday Season, and leans quite heavily towards a somewhat hostile form of atheism. However, most of those who celebrate the day seem to be good-natured and pleasant; Agnostica is not usually used as an excuse to vent spleen at the religious among us, but rather as a chance to geek out with friends and enjoy some science-oriented drinking.