Learn Portuguese (Portugal): O restaurante não abre no domingo.

In Portuguese, to say something happens "on" a certain day, you use the preposition em (in/on/at) combined with the definite article (the). Because domingo is masculine, em combines with the masculine article o to become no. Literally, you are saying "in the Sunday".

Are all days of the week masculine like domingo?

No! Only the weekend days (sábado and domingo) are masculine, which is why they use no. The weekdays (segunda-feira through sexta-feira) are all feminine. If the restaurant was closed on Monday, you would use the feminine na instead of no: na segunda-feira.

Why isn't domingo capitalized like it is in English?

Unlike in English, days of the week are not capitalized in Portuguese unless they are the very first word of a sentence.

Why does não come before the verb?

In Portuguese, negation is very simple. To say a place "does not" open, you simply put não immediately before the verb. Here, it goes right before abre (opens) to make it negative.