We're not open on Saturday nights—wouldn't that be a good time!—so I'm guessing you're asking about checking out textbooks (reserves) overnight. The basic rule is that if you check out a reserve book less than two hours before we close, it is due one hour after we open the next day (i.e. the next day that we are open). We close at 3:00 PM on Saturdays. We are closed on Sundays. We open at 7:30 AM on Mondays. So, let's put that together:
If you check out a book after 1:00 PM on Saturday, it will be due at 8:30 AM on Monday.
Of course, we have holidays sometimes, and the same rules apply: the books is due on hour after we open on the next day that we are open.