It's a day to celebrate that most mysterious and beautiful number. If everything goes right, this should show up on the blog on 3/14 at 1:59:27pm EDT. How geeky is that?
Some argue that one should celebrate at 1:59:26pm instead. Those people are Truncaters. I am a Rounder. Death to the Truncaters!

For a good intro, read John Tierney:
If this is 3/14, it must be Pi Day. In fact, it’s the 20th anniversary of the first Pi Day, a feast that began at the San Francisco Exploratorium and has been rightfully spreading in a great circle around the world.

The Exploratorium's main Pi Day site is here.

Speaking of poetry: It's not original, but here's my favorite:
Now I, even I, would celebrate in rhymes inept,
Exercise for the reader: find the hidden message!
the great immortal Syracusan rivall'd nevermore
who in his wondrous lore passed on before
left men his guidance how to circles mensurate. 
You might also
want to see the "official" Pi Day web
site. (It's nice, but … please. Official? Who said?) Don't miss the
Pi Clock, so you
can torment your friends by telling them you'll meet them at π/2 past
three.

The Wikipedia article on
π is good. For example, you can learn why, even though π's decimal
expansion is endless, learning more than a
handful of digits is unlikely to have practical application:
For example, a value truncated to 39 decimal places is sufficient to compute the circumference of any circle that fits in the observable universe to a precision comparable to the size of a hydrogen atom

There's also an entry for the "Feynman Point", which is a
good acid test to determine your True Geekiness.
The Feynman Point is the sequence of six 9s which begins at the 762^{nd} decimal place of π. It is named after physicist Richard Feynman, who once stated during a lecture he would like to memorize the digits of π until that point, so he could recite them and quip "nine nine nine nine nine nine and so on."

But in any case, whether you be
geekly or not, try to have a piece of pie today.
As long as the pie is round; despite what you learned in
school, true pie are not squared.