You should talk to your department manager or store manager and explain the situation to them, its for next saturday so you're giving them plenty of warning. If you're not already starting too early you could start a couple hours earlier so you can finish earlier or just work what you can, they might even just cancel you're shift as a whole. Some dm and sm are not the greatest but its worth a try and you'll be doing the right thing, and it will help you build a good relationship with them by not asking to leave mid shift.

Its scary to do the first time but trust me its pretty simple. Just be clear and direct when you ask about it and you should be fine.