Depends
Both "what days of the week" and "which days of the week" are correct; however, "which days of the week" is typically used when the choices are limited, while "what days of the week" is more open-ended.
Seven days in a week is more generally used than seven days in the week, except if in reference to a specific week such as 'the seven days in the week' when referring to a specific week, such as the week before Christmas.
There are 7 days in a week and there is 4 weeks in a month
5 days is one work week, but there are 7 days in a week.
There are seven days in a week.
there are 7 days in a week.
7 days in a week
7 days in a week?
There are not 5 days in a week. There are 7 days in a week. There are 5 days in the working week for most people, because we have a two day weekend.
There are 7 days in a week but only 5 days in a business week, also called a work week.
You must mean Thermopylae, but it was slightly more than two days. About a week's delay, including three days actual fighting.