Originally Posted by
Points Scrounger
Odds are that as long as you do not leave the keys behind (usual hint to staff that you no longer need them), and the room isn't listed on housekeeping's Checking Out Today roster, the cleaner will make up the room as though you'll be back, and the next day she gets a bonus (from you) in that she doesn't really have to do anything to mark the room "done" and move on to the next one.
Don't leave the keys behind, leave on Saturday morning. Housekeeping will probably assume you'll be back later that day, like
Points Scrounger said. Then, on Sunday morning, before check-out, call the hotel, tell them that you were in a hurry to get somewhere, and forgot to stop by the front desk and check-out on Sunday morning.