[Grammar] 7 days a week

[contiluo]

Which of the sentences below is acceptable? What is the difference between #1 and #2, or # and #4?

1. There are 24 hours a day.
2. There are 24 hours in a day.
3. There are 7 days a week.
4. There are 7 days in a week.

 

[teechar]

What is the difference between #1 and #2, or between 3 # and #4?

I take it that's what you meant.
Only #2 and #4 are natural.

This is because "24 hours a day" and "7 days a week" are typically used as adverbials. For example:

He sits on the sofa and watches TV 7 days a week.
This alarm system monitors the facility 24 hours a day.