Here's the thing: You (the LL) can decide that rent MUST be paid on Saturday if that is the day of the month the due date falls on. If the tenant doesn't pay, then your option for enforcement is to issue a 3-day notice. The first day you could issue that would be Sunday, but Sunday doesn't count in 3-day calculation. So, if the tenant was going to pay Monday ANYWAY, what's the point?
ETA: For practical purposes, the correct thing to do (as a business-person) would be to accept the rent on the next business day.