Once in a blue moon” is usually just a saying people use for something that rarely happens. But on May 31, the phrase will match a real event in the night sky.

A second full moon in a calendar month will occur on May 31 and be best seen at moonrise on May 30. Why is the Blue Moon one of astronomy’s most misunderstood terms?

It's called a blue moon — but this month's full moon comes with a rare twist.

A rare Blue Micromoon will appear on May 31, 2026, giving skywatchers a chance to witness one of the years most unusual lunar events. A Blue Moon refers to the second full moon…