Monumentendag literally translates to Monument Days in Amsterdam. Its one of the few times in the year where they let parts of important structures and monuments open and FREE to the public. Obviously space is limited and for more of the activities registration is required.
But if you are lucky you can see many nice things and even get a free bike or boat tour.
I’m particularly fond of the Royal Palace on Dam and the open homes where you can take a peek into the past and how wealthy Amsterdamers use to live!
check here for information on events and tickets. It’s in Dutch but try to set your browser to translate or use google translate services.