Saying something only happens “once in a blue moon” typically means it’s extremely rare. True blue moons — defined as the second full moon in a month — aren’t actually that uncommon, but if you don’t catch the one on Aug. 31, it’ll be three years before you get another chance.

Blue moons occur because our calendar months aren’t exactly in sync with the moon’s 29.5-day orbit. That means you’ll occasionally see two full moons in a single month, something that generally happens about every two and a half years.

The last time was on New Year’s Eve in 2009, and it coincided with a partial lunar eclipse that people in Europe, Asia, Africa and some parts of Alaska could see. This year, you’ll be treated to a blue moon on Friday, Aug. 31, with the moon at its fullest at 9:58 a.m. EDT.

And the next one? Mark your calendars — scientists predict it’ll occur on July 31, 2015.

[LiveScience]