Book whenever you want - but again, no more than 331 days prior to the flight. But, respectfully, if you purchase now or soon, please don't complain about the fares later, or ask if X airline gives a credit if the prices go down, or get mad at the airline when they consider any change to the tickets to be a change and want to charge you $75 or $150 (or who knows how much by next summer) each to give you a credit.