Math whizzes may have noticed something particularly pleasing about today's date. According to , June 1, 2018 (formatted as 1/6/18 in many parts of the world) is Phi Day, a date that matches the first four digits of the golden ratio.

Represented by the 21st letter of the Greek alphabet, the golden ratio, which comes out to roughly 1.618 when rounded, is the number you get when you divide a line into two parts so that the longer part divided by the smaller part is the same as the total length divided by the longer part (or simplified: When the smaller is to the larger as the larger is to the whole).

Non-mathematicians might know it better as the number that appears constantly in nature, art, and architecture. The Pyramids at Giza, Leonardo da Vinci's "Vitruvian Man," nautilus shells, sunflower seed heads, and spiral galaxies all feature the golden ratio.

The golden ratio is also closely related to the famous Fibonacci sequence. In this series of numbers beginning with zero or one, each subsequent number equals the sum of the previous two (i.e., 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, etc.). The ratio of any two successive numbers in this sequence comes very close to the golden ratio. Shapes made with Fibonacci dimensions are considered pleasing to the eye, which is why they so often appear in art, either unintentionally or by design.

Unlike other math holidays such as Pi Day, Phi Day only comes once a century. You can celebrate the once-in-a-lifetime occasion by taking a walk outside and seeing how many examples of the golden ratio you can spot in your neighborhood.