[Photo by arsheffield.]
Check it out:
- Celebrating Square Root Day
- Have a rootin’ tootin’ Square Root Day
- Prime, Composite, and Square Numbers activity
- Free, printable Square Roots Worksheets
- About the Pythagorean Theorem
- Pythagorean Puzzle — a lesson plan prototype
- 81 proofs of the Pythagorean Theorem
And coming soon: Pi Day.
Edited to add: Jonathan‘s class enjoyed extending the concept to include other products. Using the m/d/yy format (not the 4-digit year), how many times in your life has it been true that 
Denise Gaskins' Let's Play Math 