There are seven days in a/the week [closed]
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.
Similarly:
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?
Generally, your analysis is correct. Normally the refers to an identified individual or individuals, but it also has a (rather literary) use with generalisations - what you have called class properties.
However, there are more subtleties.
A cat has four legs and
The cat has four legs
both work for the general reading.
But
The star consists of hot gases.
does not work. This has only a specific reading (a particular star) and not a general reading, at least for me. I think this is because general the requires a super-class within which to distinguish the items. So (general) the cat is distinguished from dogs, horses, etc. But there is no obvious super-class within which stars are just one kind. (This may not be the reason, but it's the best thought I have had).
For the same reason
There are seven days in the week.
doesn't work for me with a general reading, though my judgment is less clear than for stars.
In your last examples,
Cats have four legs.
is fine, but
The cats have four legs.
can only be read as specific cats, not general. General the must be singular. Weeks have seven days is unusual, but perfectly grammatical.
I can see where the use of "a" and "the" might confuse you if English is your second language. In the sentences you posed as examples, "the" is used when describing a specific week, while "a" is used when the week could be any week. Therefore both sentences are correct but have slightly different meanings. The cat, star, and cats sentences are all correct for the same reason. As for the sentence "Weeks have seven days" sounding strange to you I have no idea, but it is grammatically correct.
I do not have a degree in English, but I was blessed with a grandmother who taught college level English, and lovingly corrected my grammar in my childhood.
In addition to what others have said --
There are seven days in a week.
The seven days of the week are Monday, Tuesday,...
In (1), the statement is about an arbitrary week: For all x such that x is a week, x has seven days.
In (2), the statement is about the general concept week: It is defined as a sequence of these seven days: Monday, Tuesday,....