"La oficina está cerrada los martes."
Translation:The office is closed on Tuesdays.
I put "every Tuesday" and was marked wrong because I didn't say "Tuesdays". This should be accepted.
"Los martes" means (on) Tuesdays. "El martes" means "on Tuesday." This is the same rule for the other days of the week.
I go to church on Sunday. Every Sunday. If you tell me that your office is closed on Tuesday, I assume that you mean every Tuesday. If you are only closed next Tuesday, would you not specify NEXT Tuesday?