Rotterdam is a beautiful city, but unfortunately Rotterdam’s weather is not always that beautiful. Since Rotterdam has an oceanic climate (Köppen Cfb) strongly influenced by its proximity to the North Sea to the west, with prevailing winds. Typical for this climate are mild summers and cool winters. Summers can be morderately warm and every year there will be some days that are considered tropical. In winter time it can be cold, and occasionally it snows. But especially recent years there have been winters without one flake of snow falling from the sky. Usually December, January and February are the coldest months. Temperatures reach below freezing point quite often, but usually only during night time.
The wettest months are October, November and December. At least, those are the months with the most rainy days (about 13). A month that also sees a lot of precipitation is actually August, which is together with July the warmest month. But in general these showers are because of thunder storms which can be heavy, but usually do not last long.
So when is the best time to travel to Rotterdam?
This question can be answered in a couple of ways. Do you want to enjoy nice weather without a lot of rain, visit between April and July. April still can have some cooler days, but does not see a lot of rainfall. If you’re aiming for hot temperatures and enjoy a lot of outdoor festivals and activities, visit in July or August. But do you want to make winter wonderland pictures and maybe even skate on the lakes in the parks, visit in January or early February. But you have to be lucky then. In the last decade there have only been 3 winters in which you could go ice skating on natural ice.
So in general, the best time to spend some time in Rotterdam weather-wise would be between May and July.