[Halon]

I don't work there so can't speak first hand, but I ran into a similar issue wherr my work wanted to send me somewhere this weekend. I told them no in a tactful and professional manner, I have a vacation planned and reservations already paid for. They said no problem. I work in a different environment than you, but it sounds like its their mistake not yours. I'd just tactful tell them you won't be there. Don't fake sick, just tell then you will be on vacation like you informed them of over a month ago. It's their issue to resolve, not yours.