Flux author S.T. Underdahl muses about release dates in the inaugural post of her new blog. She makes a good observation: "The term 'release date' has always struck me as funny; as if the book is something we've been holding captive, restrained by its binding."
I actually associate release dates not with books, but with CDs. I remember walking from my house to the mall to buy Pearl Jam's second album on its release date. It was a big deal. And we heard stories about the local indy record shop (which was too far to walk to) getting in trouble for selling copies of the album on Monday, instead of on the label-mandated Tuesday. What constituted "trouble," I can't imagine now, but it seemed plausible at the time.
For books, the opposite is true. With the notable exception of Harry Potter and a few other big-name books, few books are restrained to firm release dates. What's the point? Most publishers aren't in a position to whip readers into a frenzy of pent-up anticipation, especially for a first novel; rather, they count on books to be discovered. If we've got the book in our warehouse, in most cases our goal is to get it the heck out so they can be discovered. Ditto booksellers. Books don't sell if they're not out on the floor, so honoring the publisher's specified release date is about the last thing anyone, including the publisher, cares about.
So, yes, S.T., today is your release date, but I'm happy to report that a couple hundred readers have discovered and enjoyed your book for at least three weeks.