Why does Montresor tell his servants to stay in the house even though he knew he will not return until the next morning?

Montresor knew fully well and understood the character and nature  of his servants: they were a disobedient lot and would do exactly the opposite of what he orders them to do. So he orders them  to stay at home during the carnival, knowing fully well that as soon as he leaves the house, the servants will  also abandon their duties and go take part in the carnival.  Consequently, no one will be in the house when he lures Fortunato into his house to commit the diabolical deed of murdering him:  "I had told them that I should not return until the morning and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned."