What is the significance of Bitcoin Pizza Day, and when is it celebrated?
Bitcoin Pizza Day is celebrated on May 22nd. On this day in 2010, a BitcoinTalk member bought pizza in exchange for 10,000 BTC. This represents more or less the first known transaction using bitcoins for actual goods.

The original post: Laszlo announcing his purchase in 2010

The significance has been growing especially now, because... of how much that 10,000 BTC would be worth today.