Amsterdam enjoys a fairly mild climate all year round. The weather is generally similar to the UK without such extremes as sometimes exist here. Average summer temperatures are about 22°C and winter days tend to be about 5°C with nights rarely going below -5°C. Rain is possible all year but is mainly concentrated between October and March, and is generally light and short-lived. Amsterdam can experience quite large weather fluctuations between days so remember to pack some warm things even if you're going in summer.
Most people choose to take their holidays in Amsterdam during July and August when the weather is at its best. However, with better deals on Amsterdam hotels and restaurants as well as fewer tourists outside these months it's worth considering a different time of year. Mid-April to mid-May is the best time to visit the bulb fields that surround the city and throughout the winter Amsterdam's cultural scene really comes alive.