Today is Pi Day (March 14, or 3.14). Of course, we celebrated with our traditional Pizza Pi(e)s. But because Google informed me it was also the 101st birthday of Akira Yoshizawa, who is considered the grandfather of origami (see below):
I went searching for origami and circles, and chanced upon this wonderful website, WholeMovement.com. The author, Bradford Hansen-Smith, inspired in part by Buckminster Fuller, has compiled tons of information about all the mathematical and other concepts one can learn by folding circles. It doesn't take fancy equipment--he starts with paper plates and bobby pins--but it can take you deep into mathematical and geometric concepts.
So a great way to observe Pi Day (besides eating pie, pizza or otherwise) is to check out his website.