Help! Starting my first part time job soon, but I have a slight problem?

I said that I would be free to work during a certain couple of days, and this was before the interview. Now I've remembered that ages ago my sister got tickets for us to see a show on one of the days that I said I'd work. I don't think I can work earlier and leave earlier, I need to leave the shop by 5pm in order to get the train on time and everything but I think that's when my shift would start.. What should I do, would they let me have the day off? :/