Square Root Day occurs when both the month and the day of the month are the square root of the last two digits of the year.  For today, the square root of the year 16 (2016) is 4 (4th month of the year and the 4th day in April)!  There are 9 Square Root Days in each century:

1/1/01, 2/2/04, 3/3/09, 4/4/16, 5/5/25, 6/6/36, 7/7/49, 8/8/64, and 9/9/81.

One suggested way of celebrating the holiday is by eating root vegetables cut in squares!