Halloween time would be pretty fitting for Harry's triumphant return to bookstore shelves. As someone who works in an adjacent profession, I can confirm that Tuesday is New Release Day. I'm not actually sure why that is, though - seems a bit random.
It's mostly for shipping. Printers release books for distribution typically the week before a book is officially on sale, and a Tuesday release date gives stores the weekend+Monday to make sure they have adequate stock, planogram their displays, and set up. Some stores like to do midnight releases for really, really big books (Twilight was the last time my local Barnes and Noble did it, as far as I can remember, so this isn't necessarily a positive thing for society), and this gives them the opportunity to do that.
It also gives a book the best chance to get on best-seller lists. New York Times doesn't reveal how they pick what goes onto their list, but Wall Street Journal is pretty open about relying fairly heavily on Nielson BookScan, which tracks individual sales rather than bulk purchases. Having a few days to rack up sales before releasing that week's Best-Seller List is usually pretty good for a book (unless you're one of those that premiere on the BSL, which has always boggled my mind, as people really don't preorder books anymore, unless it's on Amazon and the book is big enough to have release-day delivery).
Most of this is a carryover from brick-and-mortar distribution; it's become so ingrained in the publishing industry that they're going to keep doing it, regardless of real vs. perceived advantages. Since Kindle and Nook (I'm still a Nook purist, cuz I hate the Kindle app—I want page numbers, not nonsense percentages, dang it) hit the scene and digital copies overtook physical ones, there's really little point in still releasing on Tuesdays, but they won't stop. In fact, it's probably better for publishers to do a split release; Saturday/Sunday for eBook, Tuesday for hardcovers. The margin on eBooks is substantially higher; they don't have to pay a printer for anything, and they usually charge the same (or about the same) price for the new release. I think you'd see a bunch of hardcover purists cave in for digital if they could get their hands on an earlier copy.
That said, I doubt their printers would be happy about that, and you can't go screwing over your partners like that and expect to continue doing business with them.
