Foursquare Day
Foursquare Day is held on April 16. This event in the second decade of the month April is annual. 
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4sqDay or Foursquare Day was coined by Nate Bonilla-Warford, an optometrist from Tampa, Florida on March 12, 2010 being the 4th month and the 16th being equal to four squared. The idea came to him while "thinking about new ways to promote his business". FitBy4sqDay has been a quest by foursquare users to reach new fitness goals and live in a healthier way and is an excellent opportunity for local businesses to shine.
Foursquare has more than 55 million monthly active users.
Similar holidays and events, festivals and interesting facts
Rubber Eraser Day on April 15 (Joseph Priestley discovered the eraser in 1770, using pieces of rubber imported from Brazil. Then in 1858, Hyman Lipman of Philadelphia, Pa., patented the pencil with an eraser at the end);
Take A Wild Guess Day on April 15
Stoker's Day on April 15 (is celebrated in spring, at the end of the heating season. All shifts workers gether in the boiler room to extinguish the boilers and begin to celebrate. The date of this celebration is not fixed. The local authorities decide when to start and when to finish the heating season but usually it hapends not later than April 30 if the average air temperature is above +8 for 3 days in a raw);
Wear Your Pajamas To Work Day on April 16
National Tickle Day in USA on April 16
Bat Appreciation Day on April 17
Blah Blah Blah Day on April 17 (is the opportunity to stop procrastinating and get to grips with all those stalled projects and broken promises right now);
Ford Mustang Day on April 17 (The Ford Mustang was first introduced to the world in April of 1964);
Haiku Poetry Day on April 17
Kickball Day on April 17