Monkey Day is celebrated annually on December 14th.
Monkey Day is an unofficial holiday celebrated internationally on December 14. While the holiday is mainly about monkeys, it also celebrates other non-human primates such as apes, tarsiers, and lemurs.
The holiday was started in 2000 when artist Casey Sorrow, then an art student at Michigan State University, jokingly scribbled Monkey Day on a friend’s calendar, and then first celebrated the holiday with other MSU art students. It gained notoriety when Sorrow and fellow MSU art student Eric Millikin began including Monkey Day in their artwork and Fetus-X comic strips, and began promoting it online along with other artists. Since then, Monkey Day has been celebrated internationally, across countries such as the U.S., Canada, Germany, and the United Kingdom.
This excerpt was taken from wikipedia, http://en.wikipedia.org/wiki/Monkey_Day.
For more information about Monkey Day, you can also visit http://monkeyday.com/