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.