A blue moon is a term used to describe when we see the full moon twice in a single month. The Moons cycle is 29.5 days, so just a bit shorter than the average length of a calendar month. Eventually, that gap results in a full moon happening at the beginning of a month with enough days still remaining for another full cycle ― so a second full moon in the same month. This happens every two to three years. There are two types of blue moons: monthly and seasonal. A monthly blue moon is the second full moon in a calendar month with two full moons. A seasonal blue moon is the third full moon of an astronomical season that has four full moons.

The last blue moon occurred on August 31, 2023, and it was also a supermoon, which means that the Moon reached its closest point to Earth during its orbit and was in its full phase. The next monthly blue moon will occur on August 30/31, 2023, and the next seasonal blue moon will take place on August 19/20, 2024.