The Dutch climate is strongly influenced by the wet wind coming from the North Sea. This means that winters are normally not very cold and summers are not very warm. The sea also brings wind and rain. In the western part summers are colder and there is more wind and rain than in the eastern part of the country.

Best time to visit Holland is in June. This is just before the summer holidays, so it is nice and quiet, the weather is good and you have long daytime.

In summer it may happen that you have a period of 10 days with temperatures in the range 25-30 deg C. It also may happen that temperatures does not exceed 18 deg C with rain and clouds every day.

So, make sure you have both warm clothes and summer clothes. You need to take precautions to keep your luggage dry. (I use the large grey garbage bags, you can buy in any super market. for this purpose).