Cook your pork butt at 250-275. it will get done much quicker and you will still have a quality product.
If people start showing up around 1, I would aim for my service to be about 2pm. Cooking between 250-275 a 10lb pork butt will take approx 10-12 hours, if not a little longer. I would throw the pork butt on the UDS about 10-11pm so it finishes between 9-11am.
Take if off the smoker when it is done and let it vent for a little, then wrap tight in foil and put it in a cooler or an oven that is turned off for a couple hours to rest. The pork butt should be done before anybody shows up and all you have to do when your most of your guest arrive is take it out of the foil, pull it and then add the fat that is in the foil into the pulled pork.
All my large cookouts (memorial day/labor day) I do 40-50lbs of pork butts and a brisket or two. I tell my friends that you can come over about 3pm and I'll serve the bbq when a majority of the people have arrived. My meats are typically done no later than 12:00pm that way they have ample time to rest and everything is ready to go before my guests arrive. I have yet to miss a deadline and the bbq always turns out great and my friends all rave about it.
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