Annnnnd what we have at FedEx is much better than what a lot of carriers have.

In my 3 years at AA I got 150% pay exactly once, for 3 whole hours after we got sent to JFK when we were scheduled to layover in STL (and I only got the extra pay because it put me over SPROJ for the month - the captain got straight pay).

Many people at many airlines work on days off for straight pay. There is no simple answer to the question - every contract is different.