Learn French: Le magasin est fermé le dimanche. - The shop is closed on Sundays.

In French, placing the singular definite article le before a day of the week indicates that something happens regularly on that day. Therefore, le dimanche naturally means "every Sunday" or "on Sundays".

Why isn't there a word for "on" in the French sentence, like "sur le dimanche"?

French never uses a preposition like sur (on) or en (in) with days of the week. You simply use the day by itself for a one-time event, or with le if it is a recurring event.

Is fermé a verb or an adjective in this sentence?

It is the past participle of the verb fermer (to close), but here it functions as an adjective describing the state of the shop. Since le magasin is masculine and singular, fermé takes no extra agreement endings. If the subject were feminine, like la banque (the bank), it would be fermée.

Could I say tous les dimanches instead of le dimanche?

Yes! Tous les dimanches literally means "all the Sundays" (every Sunday) and conveys the exact same meaning. Saying Le magasin est fermé tous les dimanches is perfectly natural.