If something happens once in a blue moon it happens infrequently (not very often).
A blue moon is the second full moon in a calendar month. This only happens approximately once every two and a half years.
The most recent blue moon was visible in the United States, Canada, Europe, South America and Africa on New Year's Eve 2009/2010, and in Australia and Asia on New Year's Day 2010.
Examples of use:
a) Our friends have emigrated to Italy so we see them once in a blue moon.
b) Once in a blue moon we'll have a glass of wine with our evening meal.
c) I go jogging sometimes, but only once in a blue moon.
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