So I guess if I am not around, I need to use a/the Task Scheduler to "wake" the computer shortly before the Purge Sheet is scheduled to run. This appears the only possibility unless I shut off the sleep mode feature while I am away.
Yes, that's an option.  Me, I leave my computers on.  I've run every computer I've ever owned 24/7 and they only get turned off for maintenance (air dusting).  Never had an issue with any of them; power off/on cycles are harder on a computer than just leaving them on so I wouldn't worry about leaving one on for a couple weeks provided it is in good working order.  Of course a battery backup is the only way to mitigate power outages.
Mike