I have been doing a lot of reading on possession and there still seems to be a lot of confusion amongst people. For example, two weeks' notice requires the apostrophe because it is two weeks of notice. So using that principle, wouldn't it be finals' week rather than finals week? It is the week of finals. Even more complicated, Christmas tree would never be seen to have an apostrophe, yet it is the tree of Christmas. Can anyone care to explain?
Finals is a noun referring to a series of examinations at the end of a degree course, making finals week short for "a week of final examinations" (final as in the last exams the students will take that year).
final (n.) - A series of examinations at the end of a degree course: An examination at the end of a term, school year, or particular class.
The Christmas in Christmas tree is an adjective, and it is used to indicate that it is a Christmas-themed tree. The tree does not belong to Christmas.
Christmas tree (n.) - An evergreen or artificial tree decorated with lights, tinsel, and other ornaments at Christmas.
Two weeks' notice is the notice of two weeks, just as old time's sake is the sake of old time. When the apostrophe is omitted, the words "two weeks" in "two weeks notice" seem to function no different than the words "one week" or "three day" would in their place, since the "s" in "two weeks" indicates plurality and not possession as it would, e.g., after a name, and the phrase "three day notice" is ugly and demands either an added "s" for possession or a hyphen.
On the other hand, "finals week", though it's also "the week of finals", is the week on which finals occur, and not the week of the finals. (I know I am using the italicization arbitrarily and the semantics about "of" are a little hazy, but I think it makes the point.) I think it's from the letter "s" that confusion arises. Finals are just a thing, and "finals week" is the week associated with that thing, the same way "Christmas" is a thing and "Christmas day" the day associated with it, not "Christmas's day".
A case could be made for finals and Christmas being used as adjectives, but not a very strong one.
Stuff of this kind is idiomatic. I'm beginning to hate that word, but what can I do?
Why is it ice tea but iced coffee? Go figure.