There are two defining ideas on the nature of Blue Moons. The most popular and currently used idea is that a Blue Moon is the second full moon in a month that happens to have two full moons. Since the time between two full moons is 29.5 days and the average length of a month is 30.4 then the math (29.5/(30.4 – 29.5) ~ 33) tells us that a Blue Moon should occur about every 33 months.

The next scheduled date is June 2007 but since June is a shorter month the Blue Moon is missed by a day. The next Blue Moon will be in December 2009, 30 months later. The next date is August 2012, 32 months later. The next date is July 2015, 34 months later. In 2018 there are two months with Blue Moons. The only months where this can happen is in the months preceding and following a month with less than 29.5 days in it, February. In 2018 there will be a Blue Moon in both January and March. The last time this double Blue Moon happened was in 1999. The next date is October 2020, 29 months later.

The second idea about Blue Moons states that a Blue Moon is the third full moon in a season, as defined by the solstices and equinoxes, that contains four full moons. Sky & Telescope magazine admits that the first idea may have originated from a misinterpretation of the second idea in one of its early articles. The second may have originated from difficulties in naming the full moons in such a season on the Roman Catholic calendar.

The next scheduled dates for the second idea is August 19, 2005, Feb 20, 2008 and November 21, 2010. Historically, Blue Moon days have been both happy and sad.