Rules for using hyphen against apostrophe

1.  Why is it wrong to use a hyphen in "two-week notice" when it is correct to say "five-day trip to X"?

Two-week notice

2.  On the other hand, why is it wrong to say "five days' trip to X" when it is correct to say "two weeks' notice"?

Could you please teach me when to use hyphen vs. apostrophe and vice versa?  Your help is very much appreciated.