How long did you book off?
By law, your employer simply has to give you notice that matches the period of time you booked off. So if you have only booked one day off on a Saturday, your employer can give you written notice on a Thursday that you can't take it as a days holiday and legally your request has been declined, despite previous agreements as long as there is good reason (such as staff shortages or increased demand).
I've never done this, and it stinks if your boss is doing this to you considering the circumstances, but that's the law.
If you don't turn up on Saturday it could be considered gross misconduct and you could be sacked.
How about you remind your employer that you are actually a bridesmaid and that the bride has gone to great financial expense for your hair, makeup, dress, shoes, dinner etc. which is already paid for and that no doubt she would want recompensing for her expenses should her bridesmaid be called into work when the day was booked off some time ago with agreement from everyone involved. Make out she is a real nasty piece of work
and then suggest that perhaps the other girl could have her holiday cancelled for that one day, or that you simply close you column and offer to work extra hard next week? After all, it's only one day?
Good luck!