Here's something I've learned from years of PBS.
Lets say you bid the middle of the pack for your domicile and Christmas is coming up and you really want it off.
You ask for it off in all your layers because you think that there are enough people below you that you should be awarded Christmas off.
Everyone above you asks for it off too and as PBS starts to build lines, it gives all those senior to you Christmas off. When it gets to you, Christmas now becomes a "coverage" day. It's the coverage days that are wildcards every month. You never know when they are or if you are going to get hit with it. The company needs the flying covered on that day (because it gave it off to everyone above you) so it disregards your preferences and says it needs you to work that day (the dreaded CN). The next few guys below you also get hit with these coverage days. As PBS starts to get down to the bottom 3rd of the senority list, those guys get Christmas off because it's no longer a coverage day. Every holiday, I wondered why I was working and those below me had it off.
Here's the thing. PBS doesn't know that Christmas is anything special. It's just another day that needs to be covered.
PBS sucks. Everyone starts off thinking its gonna be great but they quickly realize how little control they have over their schedules.