Pi day is celebrated on March 14th. (Again, just like Helvetica, these things do in fact exist, I am not making it up!) On this day, geeks around the world (especially at MIT) celebrate the symbol for the ratio of the circumference of a circle to its diameter. (You still here? Did I lose you? Hang on it gets better...)
Every good Pi celebration includes, well, pie.
Rhubarb, apple, pear, cherry, peach etc. March 14th is also Albert Einstein's birthday, so I suppose you can also have some birthday cake on Pi day.
Looking for a laugh? Go ask someone how they celebrated Pi day. Act shocked when they have no idea what you are talking about and then sit back and enjoy the feeling of knowing some geeky thing that others don't. (Unless of course they read this blog too.)
(Did you forget to celebrate? It's not too late. Send a e-card!)