A "blue moon" has two main meanings related to the occurrence of full moons:

• Traditional (seasonal) definition: A blue moon is the third full moon in an astronomical season that has four full moons instead of the usual three. Astronomical seasons are defined by solstices and equinoxes, and this definition comes from the Maine Farmers’ Almanac and ecclesiastical calendar traditions to keep lunar events aligned with church festivals

• Modern (monthly) definition: A blue moon is the second full moon occurring within a single calendar month. This definition originated from a misinterpretation published in a 1946 issue of Sky & Telescope magazine and has since become the more popular understanding of the term

In both cases, a blue moon is a relatively rare event, happening about once every 2 to 3 years. Is the moon ever really blue?
The moon itself is not actually blue under normal circumstances. However, it can appear bluish due to atmospheric conditions, such as after large volcanic eruptions or forest fires, when dust or smoke particles filter the light in a way that makes the moon look blue. This natural phenomenon is rare and unrelated to the calendrical definitions of a blue moon

. Summary:

• Blue moon = either the second full moon in a month or the third full moon in a season with four full moons.
• The phrase "once in a blue moon" means something that happens rarely.
• The moon can sometimes appear blue due to atmospheric effects, but this is uncommon and separate from the calendar-based blue moon meanings