Learn Spanish (Spain): ¿Qué haces el martes? - What are you doing on Tuesday?

In Spanish, you don't use a preposition like "on" before days of the week. Instead, you just use the masculine word for "the", which is el. So, el martes translates to "on Tuesday".

Does the "s" at the end of martes mean we are talking about multiple Tuesdays?

No, martes is just the singular word for Tuesday. The days of the week from Monday to Friday all naturally end in "s" in Spanish. If you wanted to say "on Tuesdays" (plural), you would change el to los and say los martes.

Why does the sentence use the present tense haces if Tuesday is in the future?

Just like in English when we say "What are you doing on Tuesday?", Spanish often uses the present tense to talk about future plans that are already scheduled or happening soon.

Where is the word for "you" in this sentence?

It is built directly into the verb haces. Haces is the "you" (informal) form of the verb hacer (to do). Because the ending already tells us exactly who is doing the action, you don't need to include the word tú (you).