There are no set rules for best time to buy flights, sometimes day they are released are best prices other times a few days before travel for Charter flights as they sell them off if any spare seats.
For August ours were released last Sept at £1,700pp, I booked 29th Jan at £1,170pp, now they are £2,300pp
Its all about knowing approx price for your dates and buying when you think they are at the lowest price, not easy.