Should I talk to a mate I’m comfortable with or the mate who writes the schedule?

I’m trying my hardest not to work on Saturday the 22nd and no one is taking my shift. Should I talk to a mate I trust or the schedule writer? I’m pretty much just gonna say that I had an obligation a couple weeks ago so I took off, then it got rescheduled for this Saturday and I didn’t have enough time to take it off before the schedule was posted.