Often when a run is reasonably underway, the leads will each have a night of the week off (or sometimes more). This is generally a regular thing and usually, if you ask at the box-ofice, they will tell you. These days it is sometimes publicised openly - as for Maria currently, and with Acorn Antiques a couple of years ago (you were told you wouldn't get Julie Walters on Mondays - though when I went Neil whatsit was off, and that wasn't publicised, though it was apparently regular for that night). Obviously, the situation changes when someone is actually ill, or something - you can't predict that. As someone said above, this sort of detail will be in the contract.