Sunday is 1/7th of the days, or 14%. In the 24/7/365 airline business we have a hell of a lot more than 14% of people in each workgroup who could claim some flavor of Xian religion and demand Sunday off. That would be disastrously bad for the ordinary shift workers who go home every night.
For the crew force, which often works 2 to 5 days straight out of town, that would mean they could no longer leave town if it entailed being gone over Sunday. For round numbers, 60% of our total workload touches a Sunday.
When the US government launched their ill-fated COVID vaccination mandate for all major government contractors and transportation providers, one of the exemptions available was a religious one. About 30% of our people claimed that exemption, which the company granted without issue or investigation. The religion most of them shared was actually FauxNewsianism, regardless of what mainstream name they put on their exemption application. This was all rendered moot when some court someplace put it all on hold, and there it sits today. But it clearly shows that when a benefit is available for claiming something inherently unprovable, a lot of people will claim that benefit.
In the paragraphs above there’s nothing magic about Sunday specifically; the same problems would obtain if everyone was able to claim Tuesday off. Except that it’s the special day of the US majority religion. Now add in a few other minority religions that treat Fri or Sat as special (or a 24-hour period that’s not a calendar day), and the havoc grows exponentially.
We’re a big enough operation that giving almost any one worker any given day off is negligible and easily accommodated. But when a hefty fraction of everybody does that we have a MAJOR problem. How to decide which employees are more deserving than others? Who will decide?
That way lies madness.