Mark your mobile calendars and get out your good badges: April 16th, 2011 is officially Foursquare day. New York Mayor Michael Bloomberg made it official in the Big Apple and 13 other cities have recognized the day, as well. Why April 16? Because it's 4 and 4 squared, or 16.

Bloomberg took some time to bestow honors on New York's own uber popular startup, even cracking a joke about the location-based social networking app's "mayor" system, by admitting that he isn't actually mayor of City Hall on Foursquare. Whoever holds that honor should probably watch their back.