My publisher and I have been discussing this. In these times of e-readers and posts on the internet when does that celebratory moment happen? The old-fashioned working definition is that a manuscript becomes a book when it's bound, so that's what we're going with. Which means, if you count the ARCs ("Advanced Reader Copy" - uncorrected versions put out six months ahead of time to get reviews and blurbs and such) then my manuscript will become a book sometime in the next four weeks.
I'm thinking it's going to be a happy new year.