The expression “once in a blue moon” means occasionally or rarely. For example, a less-than-active person goes to the gym once in a blue moon. The term “blue moon,” however, has had many different meanings over the centuries. Its usage can be traced back to a 1528 text in which it described something absurd. The term then evolved to mean never.
It is believed that at some point in the 19th century, “blue moon” took on a literal meaning: when dust from volcanic eruptions and/or smoke from forest fires in the air made the moon appear somewhat blue. These were fairly rare occurrences, hence the term “once in a blue moon” – the meaning of which has not changed.
Many people today believe that a blue moon is the second full moon in a calendar month and is part of folklore, and that it was behind the origin of the expression. However, “this is incorrect,” says folklorist Philip Hiscock of Memorial University in Canada. Unfortunately, “the false information spread with the popularity of the board game Trivial Pursuit, whose researchers took the description from a 1985 children’s almanac, which took it from a 1980 radio show, which, in turn, took it from a 1946 magazine article – where the mistake originated.”
The 1946 magazine article’s erroneous description of a blue moon was a misinterpretation of the Maine Farmers’ Almanac’s definition of a blue moon, which described a blue moon as the fourth full moon in a season that usually had three. The article’s writer wrongly assumed that if there was an extra full moon in a season, then there must have been two in one calendar month.
Source : http://www.readersdigest.com.my