Today is March 14th or 3 14 as the yanks would write it. It's the closest any date in a calendar year gets to the value of Pi. So, it's been dubbed International π Day.
As it turns out, 2006 is the 300th anniversary of the first use of the symbol π was used to represent the ratio of the circumference of a circle to its diameter, in the publication, “Synopsis Palmariorium Mathesios”, written by William Jones in 1706.
In case you're at a loss as to how to celebrate, take a look at these Pi Day Resources and, of course, the obligatory Wikipedia article on π.