If you’re waiting for something good that only happens “once in a blue moon,” today might be your lucky day.

A blue moon is traditionally considered the second full moon in a calendar month. This year, there was a full moon on July 2. Because the lunar month is 29 ½ days long, the full moon returns on July 31 — and there we have the blue moon.

However, the Maine Farmer’s Almanac introduced the term “blue moon” to describe the third full moon in a season containing four, according to The Guardian.

By that definition, the next “blue moon” is May 21, 2016, not tonight.

However you define a “blue moon,” it happens once about every 2.7 years, The Guardian reported. It’s much less rare than the common idiom “once in a blue moon” might have you believe.

Mostly clear skies are expected tonight, according to the National Weather Service. See the “blue moon” by heading outside after the moon rises at 9:06 p.m.