Tuesday, March 3, 2009, was Square Root Day, a rare holiday that occurs when the day and the month are both the square root of the last two digits of the current year. Numerically, March 3, 2009, can be expressed as 3/3/09, or mathematically as √9 = 3, or 3² = 3 × 3 = 9.
Square Root Day occurs only nine times in a century. The last one occurred on February 2, 2004, and the next will occur in seven years on April 4, 2016.
Square Root Day isn't the only humorous holiday celebrated in the math world.
Pi Day is observed each March 14 (3.14), while Pi Approximation Day falls on July 22 (roughly equal to 22/7). The first Pi Day was observed in 1988 by staff at the San Francisco Exploratorium, who walked around in circles.