As far as I know, usually cathedral must open from 6AM to 7:30PM for November to April, and from 6AM to 9PM for May to October. Even though mass/missa time, visitors can enter, but limited to access at entrance area. When I visited, the mass schecdule could be found and declared in front of main entrace. As reference, the tower is opened from 9AM to evening, closing time is upon season as 4PM for November to February, 5PM for March to April and October, 6PM for May to September. Please refer to the following website as " Zentral-Dombau-Verein zu Köln von 1842 (ZDV)" in more detail, even though not sure because I cannot read German.