AS/400 IPLs every Saturday morning, how do I stop it?

The AS/400 local to me is IPLing every Saturday morning at 4:00am.
At one time this was fine, but now we have people working swing
shift on Saturday and I need to change this to Sunday.

I can't find out where this is set!  

After it IPLs I can DSPSYSVAL QIPLDATTIM and see it it now set 
up for the time of the next Saturday.  I have tried changing this
to the next Sunday (with no effect), I have held all the jobs that
could have anything to do with IPLing in the job scheduler, I have
gone into GO BACKUP and looked at the schedule there (which are the
time I had held the jobs in the job scheduler) and I still can't find
out what is causing my AS/400 to re-ipl every Saturday!

This was set up before I came back to this company after a years 
absence, so I don't know what they did.

Any clues?  What else could cause an AS/400 to re-ipl besides a
backup?

The people working swing shift are starting to get a little snippy
about the system going down for 45 minutes every Saturday.

Regards,

Jim Langston
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