In the first paragraph of "The Black Cat," the narrator says that he will die "tomorrow," which is why he must unburden his soul by confessing his sins today. The story he tells is more of a confession of sin than crime, since most of his violence—until the story's end—was directed not against his wife but against the two black cats. It is difficult to be certain of anything in a story where the narrator is so unreliable, but it appears that he has been sentenced to death by hanging as punishment for the murder of his wife, and his execution is to take place on the day after the one on which he is writing out his confession.
The narrator's execution is foreshadowed by the white mark on the second black cat, which he claims slowly took on the appearance of a gallows. It is in moments such as this that the narrator is at his most unreliable, and he has already raised the possibility that he may be mad or dreaming, though he naturally rejects both these explanations. Whatever the reality of the situation from an objective standpoint (a standpoint which, of course, does not exist), it is clear that the narrator takes the mark as a sign of impending doom and of the form it will take.