seven
This might be more apparent to speakers of languages other than English. There is a closer match in French, for example. It is not that planets are named after the days of the week. Both the days of the week and the planets are named after various deities that have been influential in human thought over the centuries.
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.
There are 7 days in a week and there is 4 weeks in a month
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.
5 days is one work week, but there are 7 days in a week.
7 days in a week
7 days in a week?
There are seven days in a week.
there are 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.
A week is 7 days. Therefore, a week and a half would be 10.5 days.