Weather:  For fairly accurate weather forecasting: take a look at:   Weather Underground .   It also has historical data and sunrise/sunset times.

 Average Temps. (F)

 Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov 

 Dec 

 High    41  41    46  53   61  66  68   70   63  55  46  42
 Avg   36  34   39  45  53  56  61  61  55  50  41  36
 Low   34  31   35  39  44  53  54  54  50  45  38  35

Do keep in mind that these temperatures are averages! The weather in The Netherlands is highly unpredictable and could range from snow storms and icy winds during winter to heat waves during the summer and everything in between. The western part of the country tends to be less warm during summer and less cold during winter (compared to the eastern part) due to influences from the sea.

Good advice for all travelers is to pack clothes that you can layer, with a really warm coat for winter, and do not forget to bring an umbrella!