Even aliens celebrate Christmas.
New Year's Eve/Festival - Star Trek: The Original Series
Once a year at 6pm on the planet of Beta III, Festival begins. Also known as the Red Hour, this 24 period when lawlessness and debauchery reign are a release for the tranquil, violence-free society. Unlike New Year's Eve, however, their are no consequences, legally or morally, for bad decisions the next day.
Valentine's Day/Eros Day - Battlestar Galactica
Eros was one of the founding gods of the Battlestar Galactica world, leaving the human homeworld of Kobol to help found the 12 colonies. His holiday is celebration of love and affection. Much like our Valentine's Day, cards and gift are exchanged between loved ones, though the fictional holiday has a more religious bend.
Mother's Day - Futurama
Mother's Day is a celebration in which robots show their thanks to Mom, their creator. Much like children during modern day Mother's Day, robots from all over the universe get to together to send heartfelt gifts to Mom. Robots love the holiday and are known to actually buy gifts instead of stealing them.
Independence Day/Colonial Day - Battlestar Galactica
Colonial Day is an annual holiday that celebrates the official signing of the Articles of Colonization, a treaty that unified the twelve colonies. Before this, each colony was a sovereign nation but after the Cylons turned on the humans, everyone put aside their differences with this treaty. While Independence Day celebrates the end of a war, Colonial Day is more a celebration of coming together to fight a common enemy.