What is correct and why? Both, or only one of the following sentences?
1 - There are seven days in a week.
-> One single week consists of seven days. 1 week = a week = 7 days.
2 - There are seven days in the week.
-> "The week" as a general notion. The only
Week class for for all the single instances. Having 7 days is a class property, not an instance property.
A cat has four legs or The cat has four legs?
A star consists of hot gases or The star consists of hot gases?
In the last two examples I can imagine also a plural, so Cats have four legs or The cats have four legs?
As for the week, the sentence "Weeks have seven days" sounds strange. Why?