Ok I have seen this several times and I always have one question I want to asked. This is not only directed at the OP it is for anyone in simular situation. Also I read what I wrote below, it sounds rude to me. Please be assure it is not meant to be taken rudely I just could not figure how to make it sound better.
why 3 months ago when you knew the swap is coming do you not say to your boss hey I have an event on this day so I am giving three months notice that I wont be in.
I have not worked in awhile but that is what I would do and it always worked. Someone takes your schedule you take thiers. My husband does it, his employeer reworks what is being made on the schedule on days like that.
Are the employeers just that inconsiderate they wont allow time off or did ya not think of asking for the time off?