Opening hours in Japan are not extremely different as in Europe.
Most stores open at 10:00 and closes at 20:00 or later. There is no lunch break.
Stores are open on weekends but some stores close on weekdays. Especially barbers are closed on Mondays.
Restaurants and diners usually open at 11:00 except the ones which offer breakfast. Then, close at 22:00 or 23:00. Diners which do not offer alcohol usually close earlier. Then, some restaurants and diners have breaks between lunch time and dinner time. Usually lunch time finishes at 14:00 or 15:00, and then reopen at 17:00 or later. Restaurants and diners are usually open on weekends and some of them close during weekdays.
Please keep in mind when you are traveling in rural parts of Japan. Opening hours are extremely shorter than in big cities. Then, do not visit udon noodles diners in the afternoon. Better to have noodles for lunch.