"A week is seven days."
If 일주 means "one week", what does the other 일 stands for? Day? If so, the literal translation would be "one week days is seven days" or something like that? ~send help~
In Korean "7 days" is one word.
하루: one day
이틀: two days
사흘: three days
나흘: four days
닷새: five days
엿새: six days
이레: seven days
여드레: eight days
아흐레: nine days
열흘: ten days
I wouldn't recommend you to memorize this word. I have honestly never heard the word these days.
하루, 이틀, 사흘, 나흘. people say them a lot. Try the rest of them, if you have enough time or want to learn words further.
"Seven days" goes as one word, thus it's not a surprise that a word for "seven" alone doesn't appear
No one really uses those fancy numbered day words anymore besides 하루 and 이틀 in my experience