The definition of a Blue Moon as the second full moon in the same month is actually a fairly modern misinterpretation of the traditional definition, which is the third full moon in a season that has four instead of the usual three. The new definition first appeared in a 1946 edition of Sky & Telescope magazine as a result of the author misunderstanding what another author in 1943 had said about the Maine Farmer's Almanac definition. That error is now much more popular, but there was a logical reason for the traditional definition: The three full moons in each season already had names so they needed a name for the extra one, and they didn't want to give that name to the fourth one because they wanted the last full moon in each season to retain its traditional name. The May 31 moon will be a Strawberry Moon by the traditional definition, but we will have a traditional Blue Moon next May.

(My deleted post said that the moon on May 31 is a Blue Moon by both definitions, which is incorrect. I asked Google AI to list the full moons in 2026 grouped by season, and it showed four for Spring, with May 31 as the third and June 29 as the fourth. When I reread my post after hitting submit, I realized that I mindlessly stated the date Google gave for the fourth without noticing June 29 a bit late to be called a Spring moon! Anyway, there's no Blue Moon this year by the traditional definition, and none next year by the new definition.)