“Once in a blue moon”
English has lots of moon-related idioms, including ‘to be over the moon’ (to be delighted) and ‘to ask for the moon’ (make excessive and impossible demands).
The phrase once in a blue moon refers to something that doesn’t happen very often. A blue moon doesn’t actually refer to the color of the moon, but when we see a full moon twice in one month. This only happens once every two to three years. According to ecenglish.com, it’s thought that the word “blue” may have come from the now obsolete word “belewe”, which meant “to betray”; the “betrayer moon” was an additional spring full moon that would mean people would have to fast for an extra month during Lent. The saying in its present meaning is first recorded in 1821.