Adam Clarke on John 20:26

After eight days - It seems likely that this was precisely on that day se'nnight, on which Christ had appeared to them before; and from this we may learn that this was the weekly meeting of the apostles; and, though Thomas was not found at the former meeting, he was determined not to be absent from this. According to his custom, Jesus came again; for he cannot forget his promise - two or three are assembled in his name; and he has engaged to be among them.