If you go to Sunday all the npc visitors schedule will reset. I think what they are saying is if he is on Monday and you are time traveling to Tuesday and back to reset his art, if you travel to mon at 4:45 or so, you can go buy art, let the day naturally change over to Tuesday, and then time travel back to mon at 4:45am. That way you are only time traveling once per ‘trip’ instead of having to go forward and back, saving you a lot of resetting time.
Also, Redd is now guaranteed at least once every two weeks, like the rest of the visiting npcs (since the first summer update). So if you aren’t seeing him every 2 weeks, esp the person above who says they haven’t seen him since May, that seems like something’s up with your game, and maybe you should contact Nintendo to see if you have a bug or glitch if some kind.