Saki's protagonist, Norman Gortsby, believes that "Dusk, to his mind, was the hour of the defeated." Ironically, given that Norman is out at dusk, Norman is admitting that he, himself, is defeated. That said, Norman's belief instills in him a desire to watch those who
had fought and lost, who hid their fallen fortunes and dead hopes as far as possible from the scrutiny of the curious, came forth in this hour of gloaming, when their shabby clothes and bowed shoulders and unhappy eyes might pass unnoticed, or, at any rate, unrecognised.
Norman's belief that the defeated come out at dusk shows his own defeat. One can assume that his story will show one to be defeated.
That said, in order to be defeated, another must succeed. In this story, the young man is the one who is able to trick Norman. Initially, the young man's story has too many holes in it (the lack of the bar of soap and the lack of knowledge for where he is staying). In the end, the young man is able to "prove" that his story is true when Norman finds the lost soap. Because of this, Norman loans money to the young man. Based upon this, dusk is the hour of both the defeated and the tricksters.
Essentially, given that Norman's story shows both the life of the defeated and the trickster, the story "Dusk" and dusk itself is the hour where both must exist.