Explain: "It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend." Can you break down...

    I assume you are asking for an explanation of the sentence, rather than a grammatical breakdown. Please let me know if this is not correct.
    The sentence in question begins the fourth paragraph of Edgar Allan Poe's short story, "The Cask of Amontillado." It introduces the direct action of the story, preceded by three paragraphs of exposition.

It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend.

It is nearly dark when Montressor runs across his "friend," Fortunato, on the street during "carnival season." Many cities feature annual carnivals still today (especially Caribbean islands and Latin American countries), where excessive food and drink are part of the activities. Such is the case in the locale of this story. We later find that the meeting is no coincidence--it is planned by Montressor--and that Fortunato's drunken state will provide Montressor with an easier foil.