Alas, it's only pi day for folks whose date format is m/d. Folks using the y/m/d format won't have Pi Day until May 9, 3141 (3141/5/9); while d/m folks either have to pretend that January 3 (3/1) is Pi Day, or are completely out of luck (since April hath but 30 days, 31/4 is not a valid date).
Anyway, it's a grand day for engaging in circular arguments, diametric opposition, and getting round tuits. Not to mention being the day before the Ides of March. (Beware!)