A once in a blue moon event is happening soon. Here's when to look up

May's blue moon will also be a micromoon. Here's what to know.
Smithsonian Magazine

A Smaller Than Usual Blue Moon Will End the Month With a Lunar Spectacle. Here’s What to Know About the Full Micromoon

The blue moon will be the second of two full moons in the same month, a coincidence that only takes place about every two and a half years. It will also appear to be slightly smaller and dimmer than t

Once in a blue moon is getting real after two years for skywatchers and astrology followers

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.
Space.com

Why is the idea of a Blue Moon so enchanting? Over 100 years of pop culture fascination

On May 31, the lunar month culminates in a Blue Moon  — in this case, the second full moon in a calendar month. The "Blue Moon" moniker adds a touch of drama to this full moon, calling to mind the emotive associations from popular culture over the last century.