Reading Microsoft user instructions once in a blue moon may be more useful than confusing to the reader.
A blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days). July 2004 had two full moons: the first on July 2, the second on July 31 - the second full moon is called the blue moon.It's possible to have 2 blue moons in any month except February. On average, there is only one Blue Moon every 33 months.Blue Moon Dates: July 2004; May 2007; December 2009; August 2012; July 2015; January & March 2018; October 2020.
Alternative: Early issues of the Maine Farmer's Almanac described the blue moon as the third full moon in a season that has four full moons. Why would one want to identify the third full moon in a season of four full moons? The answer is complex, and has to do with the Christian ecclesiastical calendar.
Some years have an extra full moon - thirteen instead of twelve. Since the identity of the moons was important in the ecclesiastical calendar (i.e. the Paschal Moon used to be crucial for determining the date of Easter), a year with a thirteenth moon skewed the calendar, since there were names for only twelve moons. By identifying the extra (thirteenth) moon as a blue moon, the ecclesiastical calendar was able to stay on track.
Executive summary from joe-ks.com: a Blue Moon is either (A) the second full moon in a calendar month &/OR (B) thethird full Moon in a season with four full moons... In any event, it's something that doesn't happen very often!