Thanks everyone for your replies.
Looks like it varies somewhat.
I heard back form the Human Resources dept today and they told me that there are no written rules as to what hours/shifts/etc. that PRN employees have to work.
So I guess I can turn them down if I want too!
I don't know why I feel bad when they ask me to work and I tell them no.
My biggest problem is that this year, they had to cut back on personnel and I got cut about $8000 worth of income this year. So, now I'm kind of bitter that they want me to give up my holidays and work for them. Oh well, I guess I should be glad that I have a job, even though it's about half of what I worked last year.