National Monkey Day is celebrated on December 14th of each year. According to www.nationalmonkeyday.com;
The true origins of Monkey Day are deeply shrouded in mystery. Some say that the Monkey Day movement began as a tool to further the awareness of simian species and habitats. Others say that Monkey Day began as a scientific backlash to all of the religious holidays permeating the month of December, an evolutionary finger to the creationist holidays. Still others say that the concept for Monkey Day was brought on by the drunken debauchery of several young hooligans with a special affinity for monkeys, spending the first Monkey Day tauting their simian virtues and screaming obscenities at the local pub. Whatever the conception, Monkey Day has evolved beyond any simple beginnings into a greater movement.
There are many ways in which you can celebrate National Monkey Day; Dress in your favorite simian attire, talk like a monkey all day long, throw a monkey themed party, send some monkey greeting cards, draw a monkey comic, or perhaps give a monkey themed gift, or maybe even give to your favorite simian charity. Whatever you find most appropriate without being arrested.