Why does a Blue Moon happen? The moon changes phases as it orbits the Earth, according to the amount of light reaching it from the sun. The moon circles Earth every 27 days, but the geometry of the positioning of the Earth, sun and moon is the same every 29.5 days.
In other words, there are 29.5 days between one full moon and the next full moon. So it’s unusual to have two full moons in a 30- or 31-day month. Also, February, which has only 28 or 29 days (during leap years), will never have a Blue Moon. Enjoy tonight’s spectacle, because the next Blue Moon won’t happen until Oct. 31, 2020.
Read Article: https://www.livescience.com/62187-what-is-a-blue-moon.html