What does once in a blue moon mean?

Answer 1

There are in fact two definitions for a blue moon. According to the more recent definition, 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) or it means every once in a while.
A "blue moon" is when there are two full moons in one month. Since this is rare (actually once a year) the phrase "once in a blue moon" is used to say something rarely happens.
"Blue moon" appears to have been a colloquial expression long before it developed its calendrical senses. According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528: If they say the moon is blue,

We must believe that it is true. Saying the moon was blue was equivalent to saying the moon was made of green (or cream) cheese; it indicated an obvious absurdity. In the 19th century, the phrase until a blue moon developed, meaning "never." The phrase, once in a blue moon today has come to mean "every now and then" or "rarely"-whether it gained that meaning through association with the lunar event remains uncertain. Explanation for answer 1 and Answer 2: What answer one basically meant is that Blue moon comes very rarely. So Once in a blue moon means occationaly or "Every now and then"

Answer 2

"Once in a blue moon" is an idiom that means something that rarely happens or occurs, essentially implying it is very infrequent or rare. It refers to a blue moon, which is the second full moon in a calendar month, occurring roughly every 2-3 years.