Learn English idioms | Once in a blue moon
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Once in a blue moon. This is a very common expression to do something once in a blue moon means to do something very, very rarely, very occasionally, so if you do something once in a blue moon it means you almost never do it. It's not common. You could say, "I clean my house once in a blue moon" so my house is very dirty. I don't like cleaning my house, I'm very lazy, so I only clean my house once in a blue moon.
You could also say, "I don't like cooking" or "I'm not very good at cooking" so "I only cook for myself once in a blue moon". Alternatively, you could say, "I love cooking" and I don't go out to restaurants very much because I like cooking or because it's very expensive to go to restaurants so "I only go to restaurants once in a blue moon".
You can use it to talk about people as well that you don't see very much. So you could say, "I only see my mother-in-law once in a blue moon" because she lives a long way away or maybe I don't really like her! "We only see each other once in a blue moon". I don't like shopping, for example, so I could also say, "I only go shopping once in a blue moon". I try to avoid it, I don't like doing it, so it's something I only do very occasionally, very rarely, hardly ever.
So where does the expression come from? Well, it's quite an old expression and it really relates to the fact that we do sometimes get blue moons, the moon sometimes does looks blue and this is because of the weather at the time. It can be because of the air, it can be because of the light or because of the temperature, but it's not very common, it's very rare, it's very beautiful as well, but we only get a blue moon once in a blue moon.