<human>: Can you give me a shortened version of the text?

Throwaway because a few coworkers know my username.

I'm starting a new job that is a substitute position in a field that is degree-relevant. My orientation is scheduled for this coming Monday... but HR failed to tell me it spans an entire week (9-5).

I have Monday off at my current job (part-time cashier at a grocery store), but I am scheduled to work the rest of the week since I didn't know I would have more training those days. It is too late to make changes to the schedule and we are not allowed to have coworkers cover/switch shifts, so I am forced to call out Tuesday-Friday.

What excuse should I give my manager? The truth isn't an option and neither is death of a loved one. I'm thinking I'll go with an illness since everyone's getting sick with changing seasons/back to school, but I don't actually get sick often and so I don't know what would keep me out of work for an entire week.
<bot>: HR of new job didn't tell me orientation lasts all week so now I have to call out of my current job. What excuse do I give my manager?