This English expression is slightly out-dated but is still used fairly frequently. It is used as a metaphor rather than relating directly to the moon.
The expression is used to denote that something happens very rarely. Another English idiom “when pigs fly” is similar in usage although the phrase “once in a blue moon” is generally linked to things which do happen very occasionally whilst “when pigs fly” links to those things which will never happen.
The first usage of this expression dates back to 1528 in a conversation between William Roy and Jeremy Barlowe. It can be found in an anti-clerical leaflet and reads ““Yf they say the mone is blewe/We must believe that it is true.” It is suggested that the expression links to the fact that at that time it was believed that the moon could never be blue in colour and thus the term was used in the same way as the current expression “when pigs fly.” However, more recently it has been discovered that the moon can in fact be blue in colour after a volcanic eruption and under other circumstances, although this is a rare occurrence. It is for this reason that the phrase is now used to mean “very rarely” rather than “never.”
An example of the usage of this expression can be found in the title of an article in The Guardian newspaper; “Tomorrow is once in a blue moon.” This title relates literally to the blue moon but is also cleverly used in its metaphorical sense.