16 is the product of squaring four, so naturally the date of April 16th is the most logical one to be called Foursquare Day. Just why such a celebration should exist is less obvious. As it turns out, a grassroots movement of Foursquare users built up steam around the idea of dedicating a day to the online check-in service, that concept then filtered through to Foursquare's founders who decided to go "100% in on this," and now, somewhat surprisingly, so has New York City Mayor (the elected one) Mike Bloomberg. As Mike sees it, Foursquare represents a fine example of NYC's startup-friendly environment, which is why he's giving it and its heretofore informal Foursquare Day celebration the official NYC seal of approval. He even went so far as to deliver a Mayoral Proclamation to the Foursquare offices, which you can see for yourself after the break.