One of the pitfalls of shift work...
Swapping shifts can be one of the single biggest headaches of the job. When you agree to work someone else's shift the payback is when you want it.
I used to manage a department full of shiftworkers, both management and union, and shift swaps were common, but there were rules. For managemement, there was an honor system. For union workers, there were rules not only within the contract with the union, but rules I set in place. Even with the rules, the employees would whine every time they wanted to swap outside the rules.
If you were scheduled to work a major holiday, you worked it, period, no swaps. Halloween is NOT a holiday. Shift swaps must be paid back within the same calendar week. You cannot work two consecutive shifts at straight time, which means you can basically swap days off. All shift swaps must be approved in advance. And so on...
It's pathetic how union employees pick and choose what parts of the contract they want to adhere to.