Foursquare Day

Foursquare acknowledged a grass-roots effort that started in Tampa, Florida in 2010 by declaring April 16 "Foursquare Day", April being the 4th month and the 16th being equal to four squared. Some cities have made official proclamations of April 16 being Foursquare Day (Istanbul, Turkey; Atlanta, Georgia; Austin, Texas; Cincinnati, Ohio; Corpus Christi, Texas; Gaithersburg, Maryland; Indianapolis, Indiana; Kalamazoo, Michigan; Kennesaw, Georgia; Manchester, New Hampshire; New York City; Portsmouth, New Hampshire; Santo Domingo, Dominican Republic; Seattle, Washington; Miami, Florida; Victoria, Canada; Toronto, Canada; Ramat Hasharon, Israel; Singapore).
Foursquare Day was coined by Nate Bonilla-Warford, an optometrist from Tampa, Florida on March 12, 2010. The idea came to him while "thinking about new ways to promote his business"

Foursquare
Foursquare is a location-based social networking website for mobile devices, such as smartphones. Users "check in" at venues using a mobile website, text messaging or a device-specific application by selecting from a list of venues the application locates nearby. Location is based on GPS hardware in the mobile device or network location provided by the application, and the map is based on data from the OpenStreetMap project. Each check-in awards the user points and sometimes "badges". The user who checks in the most often to a venue becomes the "mayor," and users regularly vie for "mayorships."
The service was created in 2009 by Dennis Crowley and Naveen Selvadurai. Crowley had previously founded the similar project Dodgeball as his graduate thesis project in the Interactive Telecommunications Program (ITP) at New York University. Google bought Dodgeball in 2005 and shut it down in 2009, replacing it with Google Latitude. Dodgeball user interactions were based on SMS technology, rather than an application.