Shops where you can buy groceries, and supermarkets (Aldi, Lidl, Colruyt, Delhaize, Carrefour, Spar) are usually open at 8.30am and close after 6pm. Almost all shops are closed on Sundays.
You can also buy groceries from the local market. Most municipalities organise markets on a certain day through the week. The market day varies in every municipality and city.