According to Wikipedia data it rains least in April, but still a lot by Colorado standards. Most precipitation is in October and July, though I suspect that some of that is snow in the former, so my guess is July for the rainiest month. However, in Amsterdam it appears to be August (again, adjusting for snow).
It's present tense, but refers to the idea that it 'usually' rains a lot in April. It's saying that, generally speaking, a lot of rain falls in April.*
*However, I think this sentence was inspired mostly by the English expression "April showers bring may flowers," as it doesn't actually rain that much in the Netherlands in April compared to other months. See my other comment below for a more detailed explanation of the expression.