If you are an accomplished turkey breast smoker, smoke it that morning. If not I'd never smoke on the clock! You always lose when its the most important.
I would smoke it the day before and wrap it with a weeeee little bit of juice/fluid and then just warm gently in the oven the next day.
Things to look for on the breast, bone in or out, single or both sides, enhanced or not. If its already enhanced you can still brine it but it will not take too much, I usually just smoke a pre-enhanced and hope I like their brine. Please don't ask what it is that all those big words really mean though, it ain't pretty.
Everyone is different, seems folks from the north really go for that crackly skin, me I like the meat. I do low and slow with turkeys because it are notoriously known for tuff. You don't hear folks say tuff chickens or tuff pork, you hear tuff turkeys and tuff beef (I do understand C-mans cows are all happy, contented and fall off the fork tender).
Anyway, if its bone in, there is that rule of thumb used here of 4/140. That rule can be bent though with a pinch of cure in your brine, if you brine.
For the holidays I usually try to smoke low and slow, like forever. I like the holiday smoke, I like how everyone feels around the smoker, I like the way food tastes low and slow never a tuff meat. Its all about the love baby, all about the love ......
Course you can get all hot and fast wityh crackly skin too...... There is no perfect way, only the way you like it.