At least a couple of times a month, I receive a phone call or an email from an author an their book's release date asking me these questions, or something similar. The marketing guide we send to our authors does a pretty good job of explaining the significance of a book's release date, but I think many authors have built up this date in their minds into something it is not.
Frankly, I think too much is made of the book's release date. There are certain things that happen on a book's release date, but many of those are things that occur in the publisher's office and are not things that would be immediately noticeable by an author. I wish I could come up with a name other than "release date," because I think that term conjures up images of a movie premier, or people lining up around the block at midnight waiting to buy the latest "Harry Potter" book. More often than not, an author isn't going to notice anything different occuring on their book's release date then what was happening the day before that date.
With that being said, here is what happens on your book's release date, and what doesn't:
Your book likely won't be in a bookstore on this date. Some publishers have distribution deals with certain bookstore chains, and some of their new releases do appear in bookstores on this date. This usually happens with titles by bestselling authors, or with titles that the publisher really expects to do well. More often than not, the release date is simply the first date on which bookstores are able to order a title from the distributor. If they can't order a title until the release date, then bookstores certainly won't have it on the shelf on the release date...unless the author has placed the book in the store on a consignment basis.
You might have a TV or radio interview on this date...or you might not. Landing media time to push a book is something every publisher and author strives for, whether it's on the book's release date or not. However, publishers and authors have to work around the media's interest and availability. An author may have an interview or an article published about their book on a book's release date, but getting publicity any time after a book's release date is good, too. More often than not, publishers and publicists don't even start working on media opportunities until the book's release date, because the book isn't widely available until then.
Your publisher probably doesn't know how many books have sold on your book's release date. Sales are tabulated quarterly, and many book sellers and distributors pay their invoices once every 90 or 120 days.
You might have a book signing event on this date...or you might not. If bookstores can't order your title from the distributor until the book's release date, they likely won't be able to schedule a book signing event for you on the book's release date, unless they do so on a consignment basis or get the books directly from the publisher. Many authors throw their own "book release party" on their book's release date, and that's good, too. A sale is a sale, whether it's in a bookstore, a rented hall or a private residence.
There are probably a limited number of books available on the book's release date. With new authors, distributors and publishers have no idea how many copies of their title is going to sell, they they aren't going to print and stock hundreds of thousand of copies of a book (again, unless it's the new Harry Potter title). They are going to have enough on hand to meet expected demand. For the publishers who print their books in print runs, they may or may not sell all of the books in that print run. If they do sell all of the books from that print run, they go into another printing. If they don't, they are stuck with overstock, and they have lost money. Many publishers are switching to digital printing and printing books as they need them. That means they don't have to print huge numbers of books that may not sell. It's a cost-control measure, not a statement on how they expect an author's book to sell.
Bottom line: The success of a book hinges on much more than the first day it's available for sale. Work on generating consistent sales and demand for your book, not just a big launch date. Books that sell in consistent numbers month after month, year after year, will be much more successful than a book that did well its first couple of weeks out of the gate, and then fizzled.