May is ending with a rare celestial event: a Blue Moon. It will rise on the night of Saturday, May 30, into the early morning of Sunday, May 31. It’s called a Blue Moon because it’s the second full moon in a single calendar month - the first full moon was on May 1. This month features the first time two full moons have risen in the same month since August 2023, and it won't happen again until December 2028.