The NPCS still stand behind the counter on rest days. E.g. on Saturdays, the post office is closed but the staff are still standing there as if they are working. Same for Ishikawa Blacksmith, he's still at his shed working like on normal days even when his shop is closed. I feel bad for them but I think it also makes the game feel a little bit unrealistic if the staff are standing there but the shop is clearly closed.