Open Monumentendag (Heritage days) is an annual weekend, held since 1987, in the Netherlands, in which buildings open up to the public that are otherwise closed. At least, some of them or otherwise closed, not all of them. Also, not all buildings are necessarily very old.
All buildings that take part are accessible for free, so it can get very busy at some of them.
The weekend is all over the country, not only in Amsterdam.
Open Monumentend Dag