The year 2020 will see the occurrence of a rare phenomenon — the blue moon— on October 31. This is the first time that a blue moon will occur since it last appeared twice in the year 2018, on January 31 and March 31.

Although the phrase "once in a blue moon" is commonly used to describe a rare occurrence, the term, in its technical sense, is used to indicate a second Full Moon that occurs within a calendar month. In other words, the particular calendar year in question will have 13 Full Moons instead of the normal 12 (one full moon per month).