Amsterdam is a destination all-year long, but the majority of people traveling will come to the city between April and September. Prices on flights, as well as hotels will be at their highest during these months. Mid-June to mid-August are the really peak times, as people travel during the school holidays. Christmas and New Year holidays are the same.
The cheapest flights will be found outside these times, and you will avoid all the crowds as well. Off peak times are the best for visiting Amsterdam if you can arrange your schedule for it.