The setting must have been very important to Saki for this particular story. He wanted the reader to feel the mood of a city at that particular time, when the day's activities were at an end and the activities of the night were prevailing. Dusk, of course, makes it harder to see people's faces. They are shadowy figures and can be good or bad, honest or dishonest, harmless or dangerous. Among the people who only come out in the evening are predators. They want to remain as inconspicuous as possible because they themselves are in danger. The London streets were patrolled by uniformed policemen on foot. These "bobbies" were familiar with their particular beats and were on the watch for strangers and suspicious behavior.
It was some thirty minutes past six on an early March evening, and dusk had fallen heavily over the scene, dusk mitigated by some faint moonlight and many street lamps.
The young man tells Gortsby that he may be forced to sleep on the Thames embankment for the whole night unless he can find "some decent chap" to lend him enough money to rent a room. It will not only be very cold at night in early March, but it will also be dangerous, Anyone "fairly well dressed" who tried sleeping in some isolated spot by the river could get murdered. So Gortsby is being asked to save a man's life; and he isn't even being ask to give any money away but only to make a short-term loan. The young con artist says he only has a couple of pennies in his pocket, so he can't very well sit up in some all-night eatery or saloon.
Saki's story, and his story within a story, are more about the creatures of the night than about the experience of one man on one particular night. Saki is interested in depicting the shadowy demi-monde. Gortsby is a creature of the day. He shouldn't be sitting there in the gathering gloom. He is much too conspicuous to all the supplicants and predators. He will soon be going home to a warm fireplace and the security of a locked door. He must be lonely. Otherwise he wouldn't be sitting there, seeming by his relaxed, unguarded body language to invite people to sit beside him and listening to fabricated hard-luck stories.
Gortsby feels no sympathy for the night people, but like a lot of other upright citizens, he finds them interesting. He is watching all of them and speculating about them, passing judgment on them. He doesn't realize that some of them may be watching him and passing judgment on him as well. The elderly gentleman sitting beside him on the park bench may have been attracted there because he is another con man and considers Gortsby a good mark. Gortsby is well dressed, alone, young--how many such prospects could a con man find at that hour and at that time of year?
It would be hard for con men to operate at any other time than around dusk. After dark there would be no respectable citizens sitting on park benches. In the daytime most men would be at work, and that would include Saturdays when the story takes place. On Sundays there would be too many people around. The con men need anonymity, which is probably why Saki does not give them names but only calls one a young man and the other (assuming he is also a con man) an elderly gentleman. These "short-con" specialists have to operate in the dusk, just as Roald Dahl's trickster in "The Umbrella Man" has to operate only in the daytime and in the rain. Dusk helps to characterize these con men as predators, part of the night world, like jaguars and hyenas.
This question has been answered several times before :
THis answer has been answered several times before
The title of Saki's short story "Dusk" can be justified in that criminals usually do their work in darkness or near darkness as in "Dusk." Criminals are rarely seen doing criminal activity in the daylight. They want to protect their identities; therefore, they come out at dark.
In this short story, Gortsby is correct in that people who are defeated or dejected often show up at dusk. They too are protecting their identities. Many are ashamed of their failures and do not desire to be recognized.
Dusk is a perfect time to meander about without fear of recognition. All people have to come out at some point. It makes sense that those who fear being recognized for their failures would prefer to come out when it is nearly dark. Since most people have to come out sooner or later, waiting until it is nearly dark helps conceal their identities. No one will hassle them about their defeats or failures. Hopefully, no one will recognize them.
Saki wrote about the criminal attitudes and behaviors of man. He clearly understood human behavior. He realized that criminals or those who are just defeated would tend to protect themselves from others:
Many of his brilliantly crafted, deeply sarcastic pieces, however, deal with the criminal impulse of man.
At dusk, people appear as shadows. One shadow turns into another shadow. No doubt, the short story's title is justified. Near dark is a perfect time to come out without recognition.