Best day to see Cathedral

I have the opportunity to visit Chartres Cathedral from Paris either on a Sunday or Weds. Which day would be better? Is the cathedral actually open on a Sunday so we can go inside since they have morning masses there? Could somebody please give me the hours for a Sunday for the cathedral? Not a tour or the crypt or the tower, just the main part of the cathedral. I would be really upset to make the trip out there on a Sunday and not be able to enter the cathedral. I am leaving for Paris next week, so thanks for your prompt answer.