I think it's actually a good idea to do at least half the day before... less stress, and you'll have a ready supply of food at a designated time, in case there is a problem with the cooking time of the ones you are to do that day..
will reheat easier(faster) if you pre slice... make sure you put the slices back together as best as possible, kiind of rebuild the butt , so to speak..in a large trays if room allowance permits, or gallon sized bags if you dont have to room in fridge or coolers.... might want to drop a touch of apple juice in the bag/trays, as well... and if you think it's a good idea, more of the rub you used while smoking them..
do you have any other warming facilities other than your smoker to re heat???
If you do.. bout 3 hours before, put oven on lowest setting ,usually around 170* put pork in trays , covered tightly with aluminium foil... about an hour before serving,ramp heat up a bit, to maybe 225-250 and add any sauce you might want to use...
as your butts on the smoker are getting ready to come off, you will then have a ready supply of food to serve, as you sliced the remainder of your meat for the guests..
oh yea.. nothing wrong with doing a test run or two, to see how long the size trays you are using take to heat up the next day..