Monday, March 14
Celebrate the wonders of mathematics by calculating the diameter, or even the surface area, of a delicious pie. Any flavour will do, as long as it's circular.
If you prefer to write dates in the day/month/year format, and prefer expressing irrational numbers as fractions, you may celebrate on July 22.
But March 14 is Einstein's birthday, so do something visionary, whether it's pi-related or not.
And if you enjoy being a mathematical heretic, you can celebrate Tau Day on June 28 instead.