Nouns of time are the exception to the possessive rule.

1 Today's class. = the class of today.

2. Tomorrow's world = A television program/programme = the world of tomorrow  

3. A week's trip =A trip of two days. 

4. A Day's trip = A trip of one day.  

5  A month's holiday = an holiday of one month

       etc do not replace the word  "of"  with the word "for" in you mind. 

       The thing that comes after the 's is that which owns the 's although it is not  a literal/actual owning/possessing   something/somebody

       It is the same as "the cattle of John's" "the land of noah's" "the battle of Waterloo" "The statue of Liberty"  etc. 

   It originates from our ancient language.