Learn French: Dimanche, nous visitons le musée. - Sunday, we are visiting the museum.

Because this is a one-time event! In French, saying dimanche without le means "this Sunday" or "on Sunday". If you add le to make it le dimanche, it changes the meaning to a recurring habit: "every Sunday" or "on Sundays".

Why does the French use the present tense (visitons) for something happening in the future?

Just like in English when we say "we are visiting" instead of "we will visit", French often uses the present tense to talk about firm, planned events in the near future.

Are days of the week capitalized in French?

Normally, no! Days of the week are always lowercase in French. Dimanche is only capitalized here because it is the very first word of the sentence. If it were at the end, it would be written as nous visitons le musée dimanche.

Can I use visitons if we are visiting a friend instead of a museum?

No, the verb visiter is strictly for visiting places, like a museum, a house, or a city. To say you are visiting a person, French uses a completely different phrase: rendre visite à.