I'm not sure how much planning a grade skip requires, but I imagine they have to make sure there's room for your kid in the receiving class, and probably some paperwork they need to fill out to officially do the skip. Unofficially, I'd imagine you just need space and a willing teacher and kid.
As for mid-year versus next fall, my DS has a similar but not quite the same experience. He skipped first, then switched mid-year to a new district to an accelerated program. So we didn't have to deal with the questions of "why is your DS suddenly in so-and-so's class?" that you might get if you're in a small school. Since our DS was starting over with friends at the beginning of the 2nd, he hadn't really bonded hugely with anyone when we did the mid-year skip. And since the receiving class was more full of intellectual peers, he quickly made friends there and was able to move with that same class to 3rd grade, so we are glad we moved him mid-year instead of starting in the fall. He is very comfortable with his new class. If you have a situation where your DS will likely be with the same group of kids for 3rd, I'd vote for the mid-year skip just for that.
We still do stuff with some of his friends he made in kindy, though, and I'd recommend trying to keep in touch with any good friends your DS has already made. It helped our kiddo when we moved him twice to know that he still had his one good friend, even if they didn't go to school together.
