All Russian museums have 1 day off during a week. It might be Monday or Tuesday or Wednesday - depending on the museum. Moreover - once a month each museum is closed for what they call "sanitary day" - general cleaning. Which day? It depends on the museum. Planning of a program is tricky. Some museums (like Catherine palace with amber room) have a rule during a few hours each day when the entrance is open only for people with prepaid tourist vouchers and closed for all others - you will see that planning must be really careful. If its done by licensed guide who know all these tricks then you are on the safe side. If you are independent - try to check working hours carefully!