The Easter marks the beginning of the tourist season. With the tulip fields blossoming till may. Summer lasts until the end of September. The winter in Amsterdam can be bitterly cold, specially December through February.
There are many festivals throughout the year that make Amsterdam a great place to visit!
Amsterdam Dance Event, IDFA, Kings Day, Gay Pride, Grachten Festival, etc, etc.
You can look up the dates of these events at the Amsterdam Tourist Board website.