What is the origin of the idiom "once in a blue moon" and when to use it?

The idiom "once in a blue moon" has its origins in astronomy and the lunar calendar. It became popular in the 19th century and the connection between the rare blue moon and the infrequent event led to the phrase being used to express rarity in general. This phrase can be applied to a wide range of scenarios, such as rare family gatherings, infrequent visits to a particular place, a seldom-seen natural phenomenon, or something that happens only on special occasions.