How often does a blue moon occur?

It’s not just a song by Billie Holiday — it’s an actual natural phenomenon. And if you’re curious about the blue moon, specifically how often it occurs, you’re not alone. Because the term “once in a blue moon” is around for a reason — they’re pretty rare.

According to Space.com, they occur about once every 2.7 years. And Time and Date noted that having a double blue moon only happens around three to five times within a century.

But perhaps one of the strangest things about a blue moon is the fact that it has two definitions — and the second one reportedly came about due to an error that an astronomer named James Hugh Pruett made. (Hey, we all make mistakes.) Back in 1946, he referred to the term as the second full moon within one month. Actually, the blue moon happens when a season has an extra full moon, with 13 happening within the span of the year instead of 12. It sounds a little confusing, but makes sense if you’re interested in charting moon phases.

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This month’s first full moon happened on January 2nd, making the moon on the 31st a “blue moon” — and not just that, but a super blue blood moon, as it’ll be the combination of a supermoon, a blue moon, and a blood moon.

You’ll see the next one in March, and then we won’t see another blue moon for the rest of the year. So this is definitely an extra special time you’re interested in astrology.