What it means
You say once in a blue moon when something barely ever happens. It’s for rare stuff, the kind you’re not holding your breath for. It can sound warm, jokey, or a bit side-eye, especially if you’re calling out someone who only appears when the planets feel generous.
Usage examples
"She replies to the group chat once in a blue moon, then shows up like nothing happened and expects everyone to have saved her a seat."
"We only get the whole family together once in a blue moon these days."
"He cleans his flat once in a blue moon, then acts shocked when he finds mugs growing ecosystems."
"My cousin posts on Instagram once in a blue moon, and every time it’s twelve blurry holiday pics in one dump."
"My brother rings the family chat once in a blue moon, usually when he needs a lift or wants someone to remember he's alive."
Where it comes from
It comes from blue moon, which has long been used for something rare or unlikely. By the 1800s, once in a blue moon was already doing the job it still does now, meaning hardly ever. The moon bit gives it that nice dramatic sparkle for stuff that barely shows its face.
