Pythagorean Theorem Day or Pythagoras Theorem Day is celebrated when the sum of the squares of the first two digits in a date equals the square of the last digit in the date.
The holiday gets its name from Pythagoras' Theorem, which defines the relationship between the sides of a right triangle or a right-angled triangle – a triangle where one angle is at 90 degrees. According to the Theorem, in a right triangle, the square of the hypotenuse (the longest side of the triangle) is equal to the sum of the square of the other two sides. The Theorem is named after Greek mathematician and philosopher, Pythagoras, who is thought to have formulated the proof for it.
The holiday is also sometimes called Right Triangle Day or Pythagoras Day.