I was in bed for seven whole days because of a cold.
I visit our family grave on the 7th of every month.
I left the company after only seven days.
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.