No, not everything is open in Barcelona on a Sunday, as the city follows specific commercial laws that vary throughout the year. Traditionally, most high-street shops and supermarkets are closed on Sundays to observe a day of rest. However, from mid-May to mid-September, the city allows shops in designated "tourist zones" (like the Gothic Quarter and Eixample) to open between 12:00 PM and 8:00 PM. Outside of this summer window, the only major shopping mall that remains open 365 days a year is Maremagnum, located at the end of the Ramblas in the port area. Fortunately for travelers, museums, major tourist attractions (like the Sagrada Família), and almost all restaurants and cafes remain open on Sundays. If you need groceries, look for smaller "Supermercat" convenience stores, which often remain open when the larger chains are closed. Always check the specific hours for any boutique shops, as they may remain closed even during the summer opening season.