(CNN) May's full moon is typically known as the flower moon because of all the blooms this month. This year it coincides with a rare occurrence: a blue moon.

Of course, Saturday's moon won't actually be blue. And the reason for the name is a little confusing.

A blue moon is the second of two full moons in a month -- but it can also be the third of four full moons during a single season. In this case, it's the third of four full moons this spring. These don't occur as often as two full moons in a month.

"The name 'Blue Moon' has nothing to do with the color of the Moon," Gianluca Masi, astrophysicist and director of the Virtual Telescope Project in Rome, wrote in an email. "Every two to three years we have 13 full Moons within a year. This way, we can have four full Moons during a given season or two full Moons in a given month."

This is the last seasonal blue moon until August 2021.

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