For the highest peak seasons, it is never best to book far in advance - your best times are between 45-100 days before day of flight. The reason is that for the highest peak times, the lowest prices are typically blocked to push determined travelers into higher price points. Then, when they have to fill the plane, they reduce the price. So - if you are buying non-refundable - generally never really great to book more than 6 months ahead for the most peak dates.