once in a blue moon
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used for saying that something can only happen on very rare occasions
The idiom "once in a blue moon" has its origins in astronomy and the lunar calendar. It became popular in the 19th century and the connection between the rare blue moon and the infrequent event led to the phrase being used to express rarity in general. This phrase can be applied to a wide range of scenarios, such as rare family gatherings, infrequent visits to a particular place, a seldom-seen natural phenomenon, or something that happens only on special occasions.
- Once in a blue moon, she treats herself to a spa day for some much-needed relaxation.
- They visit their grandparents' farm once in a blue moon to reconnect with their rural roots.
- Once in a blue moon, he cooks a gourmet meal to impress his friends.
- We went on a vacation together once in a blue moon, and it was a memorable experience.
- Once in a blue moon, they attended a concert by their favorite band when they were in town.