Blue Moon
Note that due to the 29-day lunar cycle the exact dates of the full moon move every year. Most seasons have three full moons, but because of the variation some seasons have four full moons. The term “blue moon” was used to identify one of these extra full moons. A mistaken definition in the March 1946 edition of Sky and Telescope magazine claimed the blue moon fell on the second full moon of the calendar month. This mistake caused widespread misunderstanding until it was finally corrected in 1999.