Seventh day or day seven. No matter how you look at this word, it makes a lot of sense.

Words that count days use an exceptional reading. Even though this is a jukugo word, it uses kun'yomi readings for both the kanji. Think about this word this way:

You've been stuck inside working for seven days and today you finally have a day off. Your friend asks you if you want to go for a drive but you say, "Nah, no car today" (なのか), because you've been cooped up long enough. Seven days is too long to be inside.