How do you say blue moon in dutch?

There isn't a direct translation for "blue moon" in Dutch that captures the astronomical meaning (a second full moon in a single calendar month). You would have to describe it, for example:

* Een tweede volle maan in één maand: This literally translates to "a second full moon in one month." This is the most accurate and clear way to explain it.

* Een blauwe maan (maar dan niet letterlijk): This translates to "a blue moon (but not literally)." This would be used if you want to use the English term but clarify that you don't mean the moon actually appearing blue.