Like most major museum complexes, the Vatican Museums periodically close individual rooms or galleries along the visitor route for conservation, restoration, or preparation of temporary displays, even on days when the museums are otherwise fully open.
Sundays and religious holidays
The Vatican Museums are closed on Sundays (except for a free last-Sunday-of-the-month opening) and on a number of Catholic holidays each year — worth checking the official calendar before planning a specific date.
Partial route changes
Occasionally a specific gallery is temporarily removed from the standard route, with the path adjusted around it rather than the museums closing entirely.
Check before booking
The Vatican Museums' official ticketing site lists current opening days and any known closures, and is the most reliable source shortly before a visit.