@Drun Well, it's probably one of those things that I should know but don't but for the record, I was with a Bulgarian friend who also didn't assume that because it was Monday the museum would be closed. In contrast, the Smithsonian Museums in Washington DC are open 7 days a week and for better or worse, as an adult these are the museums that I'm most familiar with. I also spent 10 days in London around the New Year and those museums were also open every day. So there you have it--a couple of fairly logical reasons for why I assumed the museum would be open.