"After Twenty Years" is about an appointment between two friends. What is special about this appointment?

In O. Henry's story "After Twenty Years," two men are meeting because twenty years previously they set a date to meet up with each other, no matter where they were living or what they were doing. Bob and Jimmy went their separate ways as young men, with Bob heading out West to seek his fortune and Jimmy remaining in New York City. Although the two corresponded a few times in the beginning, they have not heard from each other for at least eighteen years. Neither knows what the other has been doing, what his profession is, or whether he is even still alive. Yet each man feels a very strong commitment to keep the appointment, even though Bob comes from "a thousand miles" away to do so and is putting himself at great risk. For Jimmy, keeping the appointment is quite easy since he has stayed in the area where they intend to meet; in fact, as a policeman he walks a beat in that very precinct.

Bob is convinced that his friend will arrive for the appointment because he knows Jimmy is "the truest, staunchest old chap in the world." Ironically, Bob's knowledge of Jimmy's character should have warned him that Jimmy may have ended up in police work and/or would end up turning him over as a wanted criminal; instead, he believes his trip will have been worth it if his "old partner turns up." Whether he retains that opinion after Jimmy has him arrested is left to the reader to determine. Nevertheless, the fact that Bob keeps his word to his friend about the appointment and speaks so highly of him makes the reader consider that "Silky Bob" may have some redeeming qualities.