A friend offered you a beer, but you told him that you don't drink beer. You want to explain that you do have beer sometimes, but not very often. You say:
I'll have one once in a blue moon.
This phrase describes a situation that sometimes happens. It uses the future tense "will", but it doesn't describe something that happens in the future. Instead, it's something that has happened in the past several times and might happen again. In the example above, it means that the speaker drinks beer on very rare occasions - for example, once a year.
Here's another example of "will" used to describe something that sometimes happens:
"Once in a blue moon" is an English idiom that means "rarely" or "not often at all". It's a phrase that's used every now and then in friendly casual conversations to add some color or interest to the conversation.
The phrase "once in a blue moon" comes from an old expression that most English speakers don't actually know. A "blue moon" is when the moon is completely full twice in the same month. Since the moon becomes full about every 28 or 29 days and the calendar month can be 30 or 31 days, sometimes there are two full moons in one month. The second one is technically called the "blue moon". This happens a little less than once a year.
These days, most people only use the phrase "blue moon" in the expression "once in a blue moon", meaning "rarely". It can mean anything from a few times a year, to only once or twice in a person's lifetime.
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