Once in a blue moon means for something to happen very rarely.
This term is something of a misnomer (misnomer – a wrong or inaccurate name), because an actual blue moon—that is, the appearance of a second full moon in the same calendar month—occurs every 32 months or so.
Fun Fact! – The moon can appear blue in color at any time, depending on weather conditions.
e.g. “We only see our daughter once in a blue moon.”
e.g. “I don’t go to the beach often, only once in a blue moon.”
e.g. “I go to the cinema once in a blue moon!“