Reheating pork loin roast for a party
What I personally do is the following:

Note: I assume you want it well done and not rare. I would not recommend cooking the loin in 2 shots if you want it rare. 

First, when cooking the day before, stop cooking the peice before you consider it cook as you would eat it. i.e. If your cook time for done is 1h, cook it 50 minutes.

Note: for best flavor, I highly recommend you keep the fond

On the re-heat day:


Let the meat sit at room teperature for 2 hours.
Preheat the oven or bbq to 300f
In a large enough cookware with a lid or alluminium pan (I would double it up to prevent spill) and air thight foil as lid, put a layer of 1-2 cm of 50% sauce + 50% water + some butter or olive oil.
With a spoon, pour some liquid mixture you just created over the piece.
Oven it for 30 minutes, then redo the liquid pouring step while making sure it's wet enough, it not, add some more water.
Depending on the size of the piece it should take 1-2 hours to be perfect.