There isn't a good answer to the question when it is best to visit the Netherlands. Most of the time, it is cold and rainy. This occasionally gets replaced by an oppressive warmth - oppressive mainly for the fact that A/C is virtually non-existent. This isn't to say that there aren't a few good days with proper sunshine, but they're unpredictable and limited. 

Clothing 

Wear several layers and bring something waterproof.  While the rain isn't as bad as some would say, you might want to bring along an umbrella when heading out as showers come unexpected. Do note that it isn't unusual to have a couple of very warm days followed by chilly, windy weather. The Dutch say there is no such thing as bad weather, only a bad preparation.

Temperatures 

For those wanting to get a more accurate description: winter starts around the 21. December and ends around the 21. March. During winter temperatures could get below zero degrees Celsius, but minus ten (or lower) is very rare. Summer starts around the 21. June and ends around the 21. September. In summer, temperatures could get up to 25 - 30 degrees Celsius. However, most of the time temperatures vary between 15 - 25 degrees Celsius during summer and 0 - 10 degrees Celsius during winter.