Square Root Day is an unofficial holiday celebrated on days when both the day of the month and the month are the square root of the last two digits of the year.^[1] For example, the last Square Root Day was March 3, 2009 (3/3/09), and the next Square Root Day will be April 4, 2016 (4/4/16). The final Square Root Day of the century will occur on September 9, 2081. Square Root Days fall upon the same nine dates each century.

Ron Gordon, a Redwood City, California high school teacher, created the first Square Root Day for September 9, 1981 (9/9/81). Gordon remains the holiday's publicist, sending news releases to world media outlets.^[2] Gordon's daughter set up a Facebook group where people can share how they were celebrating the day.^[3]