Why do managers always schedule your first shift during their busiest day of the week?

I just got a new job offer for a food service job and my first day is Saturday, which they told me is their busiest day of the week. Almost every job ive ever had does this. Why have a newbie come in for their first shift when its so busy? I need to learn the job but doing this just makes me feel even more nervous than i already was and i cant contribute or learn anything properly anyways. I always feel like im in everyone elses way when this happens. I dont know if its just me or not but i felt like saying/posting something will make me less nervous.

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