On the 10th and the 11th of September, you can view about 30 monuments for free in Leiden during the Open Monument Days.

The international theme of 2022 in Sustainable Heritage. A theme which can mean a lot of different things in Leiden. What is more sustainable than a monument that's several centuries old, right? Well, maybe not. Monuments can still get a lot more sustainable than they are right now. Think about better insulation or double windows. The use of environmentally friendly and recycled building materials in the restoration world could also help.
From August onwards, you can find the entire program of the Open Monument Days on their website.

Location: At over 30 monuments scattered through Leiden (including themed walks and bicycle routes).
Time: Saturday the 10th of September 12:00 -17.00 | Sunday the 11th of September 10.00 - 17.00 

Entry:  Free

There's a Dutch booklet with more information about the program, and an English flyer. In some of the monuments, following an English tour is an option. 

For more information visit the Open Monument Days website.