In the story, "The Open Window," Mrs Sappleton begins to talk about her husband and her brothers. Then Framton thinks it was "an unfortunate...

It seems to Framton Nuttel to be a coincidence that he happens to be paying a visit to the Sappletons on an anniversary of the deaths of Mrs. Sappleton's husband and two brothers. Framton feels it is unfortunate because he is a perfect stranger and is imposing on her at a time when she must be remembering the tragic event because of its being an anniversary. According to Vera's story, her aunt has been expecting...

It seems to Framton Nuttel to be a coincidence that he happens to be paying a visit to the Sappletons on an anniversary of the deaths of Mrs. Sappleton's husband and two brothers. Framton feels it is unfortunate because he is a perfect stranger and is imposing on her at a time when she must be remembering the tragic event because of its being an anniversary. According to Vera's story, her aunt has been expecting the three hunters to return for tea every evening for the past three years. Framton feels like an intruder. This is what is unfortunate for him. It is also unfortunate for Mrs. Sappleton because she is compelled as a hostess to give him some of her attention when she would obviously prefer to keep looking out the open window for the return of her husband and brothers.

...he was conscious that his hostess was giving him only a fragment of her attention, and her eyes were constantly straying past him to the open window and the lawn beyond. 

According to Vera, her aunt leaves the tall window open every night until late. Vera knows that her aunt will say the same things tonight as she does every other night. There really is no coincidence. It is not an anniversary of the men's disappearance because they have only been gone for one afternoon. Vera colors her story by making this the third anniversary of their "disappearance" because it may seem somewhat more likely that they should return from the dead on an anniversary of their deaths than at some other arbitrary time. Saki has his character Framton Nuttel reflect that it is an "unfortunate coincidence" that he should be there on the third anniversary because he wants the reader to see that the falsehood has made an impression on the nervous visitor. The men's apparent resurrection on the third anniversary of their deaths is just one more factor that makes it seem plausible--both to Framton and to the reader. If men are to return from the dead, wouldn't they do so on some significant occasion such as an anniversary? Vera must think so; otherwise she would not have made up that detail.