Work problem help asap!

I usually work every Friday morning and have been for the past two months. My sister in law is having a surprise party for her husband to reveal whether she is having a boy or a girl and he has no idea any of us are going to be there. Until this Friday which is tomorrow they schedule me the evening shift at the time when the party starts and I thought I was scheduled the morning shift as I usually am. Is there any way I can get out of this one? I want to be a part of my sisters celebration so bad!!!!!