I am sure that you can book this far in advance, however, keep in mind that they will charge your credit card right away, and you risk the chance that the weather will be bad. You will be doing some outside walking, which is bad when the weather is not cooperating. My husband and I did the tour last February, and booked it the day before(president's week so the parks were busy!). They do limit the number of people who can take the tour, but they have several tours per day, and booking close to your date not only keeps your money in your pocket longer but insures you a good weather day. We loved the tour, and will probably do it again in a couple of years. Hope this info helps, and have a great time!
Laura