Granted March 14th (3/14) comes every year, so Pi Day isn't as special as Square Root Day (3/3/09) or February 13th, 2009 when we passed Unix time 123456789, but its still cause for celebration with geek friends and a tasty dessert!
There's even an official (?) Pi Day website, from which I learned the #1 way to celebrate Pi Day is to "Bake a Pie with the symbol Pi on it, measure its diameter, calculate its circumference, and eat the Pie."
Now that's my kind of holiday! Happy Pi Day, everyone! And remember, there's only six years to go before Ultra Pi Day (I just made that up) on 3/14/15!