Defrosting approx 3.5kg of pork, and then cooking more than once?
Your proposed method is possibly dangerous, you'd be warming it enough for botulism to thrive, but not enough to kill it. When you cook it, cook it properly and all at once. What I'd suggest is simply cooking it on Friday, then re-heating it saturday, or getting up early enough to cook it through on Saturday. As for when to thaw it if it's a truly heroic size I'd thaw it in the fridge starting 48 hours before you want to cook it. 

As for how long I'd usually do 20 minutes per pound plus 20 minutes as long as the meat is room temperature to start with, so for 7.5 lbs, so that's near enough 3 hours as makes little difference. Not that long really, if you wanted to serve at 1pm you'd want to get it out of the fridge as early in the day as you can, get the oven on at 8:40 on 350F (375 F on a conventional oven), get the meat in at 9, roast till 12, then rest it for an hour. Crank the oven up to max for the last 20 minutes to get the crackling extra crispy.