Pi Day is celebrated on the 14th of March (3.14) around the world. Now it is also International Mathematics Day. Since Pi is the iconic number of mathematics, there are plenty of websites & blogs, and zillions of activities out there to celebrate this day.
Here are some of the activities I have used with the Middle School and Elementary School Students;
Click here to download the "Calculating Pi like Archimedes" Poster (I recommend a giant size on-floor poster) as a PDF file.