A full Moon happens about once every 29.5 days, so you usually see 12 full Moons a year, and some years have 13.

How often is there a full Moon?

  • The Moon’s phase cycle (new Moon back to new Moon) is called a synodic month and lasts about 29.5 days.
  • Because of this, a full Moon appears roughly every 29.5 days, which works out to 12–13 full Moons in a calendar year.

Why the timing isn’t exact

  • Calendar months are 28–31 days long, so the full Moon date shifts a bit each month instead of staying on a fixed day.
  • When a full Moon falls in the first day or two of a month, there can be a second full Moon before that month ends.

Blue Moons and “extra” full Moons

  • The “extra” full Moon that appears about every 2.5–3 years is often called a blue Moon, leading to some years having 13 full Moons instead of 12.
  • Traditionally, this meant either the third full Moon in a season with four, or more popularly, the second full Moon in a single calendar month.

Information gathered from public forums or data available on the internet and portrayed here.