Yes you can and I highly suggest you do so! We are going next November, 370 days from now, and I just looked and I would not be able to get a site for the entire length of my stay now. Really glad I booked back in August!
If you know what dates you want book it as soon as your window opens. Someone can correct me if I'm wrong but you get up to 5 days before your trip to cancel for a full refund. Better to book and have to cancel than to not book and not be able to get a site.