The Prinsengrachtconert is the highlight of the annual Grachten Festival. It is a free, open-air classical music concert, performed from floating pontoons that are placed in the Prinsengracht canal.
It is always held in the evening, and starts around 8 or 9 pm (check the Grachtenfestival website for details).
Nowadays, the concert is broadcast live on national tv, but it still attracts thousands of spectators, and hundreds of boats that try to come as close as possible. The atmosphere is always very festive and romantic, with the tower of the Westerkerk church on the background.
If you want a good view, come early!