Semana (Spanish for “week”) comes from the Latin septimana for the same. Septimana itself comes from the Latin root septem meaning… seven. There are, after all, seven days in the week–by definition!
From the same root, we get the English September. But something isn’t right. Isn’t September the ninth month, not the seventh month? Huh?
The fascinating explanation is that the ancient calendar had ten months, the first of which is… March. So, the numbering is all two behind. This explains not only why September is two off, but so is October (from the root oct- meaning “eight”, not “ten”) as well as November (nov- for “nine”, not “eleven”) and December (dec- for “ten”, not “twelve.”)