Apparently, Tau Day occurs during the last week of June. Vihart of the Youtubes made a catchy tune and video to explain. ...Or I can explain here too. According to Tau, if you have a circle with a radius of 1, the circumference will be 6.283185... or something like that.
Vihart's video called, A Song About A Circle Constant, is really quite cute. She uses coloured sharpies to write out the lyrics and create whimsical stick figures which explain the concept. It's really quite nifty.