Is this a statement of fact "There are seven days in the week" or a reply to a question "Do you run often? Seven days a week!"?
The latter. The first would be 'Det är sju dagar i veckan' or 'Veckan har sju dagar' or something like that.
The literal translation is "Seven days in a week". While this is not exactly idiomatic English, I don't think it is wrong. Should it not be accepted?
The literal translation really is "Seven days in the week". In Spanish, we would say Siete días a la semana that is an almost literal translation.