After a January with two full moons and a February without any full moon, March will see its second full moon - a blue moon - on Saturday, March 31. The peak will come at 8:37 a.m.

As the second full moon in a calendar month, the March 31 full moon earns the name of blue moon.

The first full moon this month was on March 2.

There won't be another blue moon until October 31, 2020.

Blue moons generally happen about every 2.7 years, because the number of days in a lunation, which is the span from new moon to new moon, is 29.53 days, just a bit shorter than the 30 or 31 days in a calendar month - 28 days in February. A year's worth of lunations totals 354.36 days versus the 365.24 days in a calendar year, and that difference builds until there are 13 lunations in a year, producing what we call a blue moon.

The term blue moon is a derivation from an Old English word for betrayer. The moon does not turn blue.

Native Americans knew the fourth full moon of the year as the Sap Moon, the time of year when the sap was flowering in the trees.