Montressor is no fool; he knows human nature and psychology. Because it is Carnival, and everyone wants to participate in the festivities, he shrewdly tells his servants that he will be away from town overnight and orders them not to leave.
Of course, knowing people the way he does, Montressor fully expects his servants to vacate the premisis as soon as he is a safe distance from the house. And he is of course right in this assumption. After all, when the cat's away the mice will play, will they not?
Just like anyone whose parents have set limits, the servants take the first opportunity to get away with breaking their authority's rules. They think they can figuratively get away with murder when the Lord of the house isn't around, but it is Montressor who, in fact, is planning to get away with murder--literally.
Poe uses many things for double purposes. When Montresor tells his servants he will not return until morning and gives them explicit orders not to stir from the house, he knows they will all disappear as soon as his back is turned. This rids the house of servants, so there will be no one to see that he has brought Fortunato home with him. But it also is a sign of Montresor's poverty. He can only afford inferior-quality servants, and he may not be paying them regularly. They do pretty much as they please because they know he can't fire them, and they wouldn't care if he did. With the servants gone, the palazzo is empty. This shows that Montresor is all alone in the world. Poe shows Montresor's knowledge of human nature, his use of reverse psychology, his poverty, his precarious social position, and the absence of loved ones in his life.
It is festival time. He tells them that he won't be back until morning, knowing that they'll go out to party since he won't be there. He's right.
Montressor empties his house that is full of servants by saying he won't be back.
So he is basically saying even if they don't follow his instructions, Montressor won't be there to check up if they are gone.
He ends up coming back but the servants ended up leaving because they believed his words.