A Blue Moon is visible behind Castel del Monte, photographed 18 km from the castle, between Canosa and Andria, Italy, on May 31, 2026. The Blue Moon of May 31, 2026, is the second full moon of the same month. Astronomers use the term ''Blue Moon'' to refer to this ''extra'' full moon in the calendar. The name derives from the English expression ''once in a blue moon,'' which refers to something very rare. (Photo by Davide Pischettola/NurPhoto) (Photo by Davide Pischettola / NurPhoto via AFP)