DC offered a special early booking rate at New Year's 2014. While summer was blacked out, I know September was included in the promotional rate. Unfortunately, no one knows if they will offer a similar special again this year for 2015.
As a general rule, prices for DC only get higher as time goes on, so booking early is almost always preferable. We booked our August 2014 tickets in October 2013, at $249 per person. By the time we travelled, rates for the same day (which was eventually a sell-out completely) had slowly inched up in small increments to $349 pp. $100 per person is alot when you mutiply that by a family of 4 or 5.
I would book now and then keep an eye out for promotions early in the year (sign up for their email notices via their website, and Like them on Facebook). Then, if your date decreases in price with one of those sales, call them. They can cancel your old booking and rebook at the new rate, refunding you the difference, as long as you are more than 30 days away from your travel date. We have done this before, and DC's customer service have been very friendly an accommodating.