I know that most of what I've talked about in here is the mechanics of completing a book, but I was having a conversation earlier this week about publishing deadlines, and thought I should at least mention something, here:
Books take time.
Not just the writing and editing and formatting and all that, but the printing, too. It all takes time.
Sure, on some sites you can almost-instantly go from uploading your manuscript to someone being able to buy it, but whether you're working with a download or an actual physical book there is going to be some time between those two steps for things like file verifications and maybe even proofing.
If you're going a more traditional route - either through a "traditional" publisher or one of the self-publishing/publishing-service-provider companies, it can be months (or even years) from the time you start to the time that you have a finished product.
If you want books for Christmas, you typically can't just start in October. In fact, if you want your "traditionally" published book to be online - or possibly even in stores - for the Christmas season, some calendars would say that you have to have them completed and ready to leave the warehouse by as early as July. And this is true even if the book's publication date is December.
My big advice to you, then, is to look at your calendar and see when you want your books. (Do you want to have them on sale in time to become Holiday gifts? Are you planning to hand them out at the next family reunion?) Then do your research into your possible different methods for getting them in hand and start working backward from there.
If you find that the publisher you really like can't meet your deadline, you're going to want to decide which of those two things is more important to you and proceed accordingly (by changing one or the other of them).
Do you need people to have access to your work ASAP? Maybe an online ebook seller is right for you.
Have all the time in the world, because you were smart and didn't try to make plans before you knew your deadlines? Feel free to consider longer processes.
Above all - as with anything else in publishing - knowing what you want, what you must have, and what you can live without will be the first step in a successful publishing process.