Click stamp to enlarge.
It's 3/14 = 14 March, and everywhere in the universe where sentients have (on average) ten fingers, it's Pi Day!
On Earth, lots of people celebrate by baking and eating pi(e)! (I like blueberry.)
In lots of schools, students hold contests to see who can correctly memorize the most digits of the decimal expansion of π.
The usual -- but by no means the only -- definition of π is the circumference of a circle divided by the diameter. That works out to a little more than 3.14 .
But π isn't always defined in terms of geometry or circles. π pops up all over the place in pure mathematics, often in the most unexpected and mysterious places.
2015 -- next year -- Pi Day will be extra special, because 3/14/15 will be the first 5 digits of the decimal expansion. There should be some extra big Pi Festivals and Celebrations.
Click Here for tons of stuff about Pi Day.