A popular phrase, Once in a Blue Moon, is normally used incorrectly. Popular myth says it occurs when a full moon presents itself twice in the same month—like tonight—but not so, according to this article in Sky Tonight. It seems the correct use of the phrase is a more complicated formula.
SkyTonight.com – Moon – What’s a Blue Moon?
Recent decades have seen widespread popular embrace of the idea that when a calendar month contains two full Moons, the second one is called a “Blue Moon.” The unusual pattern of lunar phases in early 1999 — two full Moons each in January and March, and none at all in February — triggered a groundswell of public interest. Countless newspapers and radio and TV stations ran stories about Blue Moons.
In an article “Once in a Blue Moon”, folklorist Philip Hiscock traced the calendrical meaning of the term “Blue Moon” to the Maine Farmers’ Almanac for 1937. But a page from that almanac belies the second-full-Moon-in-a-month interpretation.
With help from Margaret Vaverek (Southwest Texas State University) and several other librarians, we have now obtained more than 40 editions of the Maine Farmers’ Almanac from the period 1819 to 1962. These refer to more than a dozen Blue Moons, and not one of them is the second full Moon in a month. What’s going on here?