I don't have a suggestion as to when to book your flight (my wedding is in January (later end) and we haven't booked ours yet), but I do suggest checking out
Airfare Watchdog to 'watch' the prices.
For example, the going rate for airfare to miami is about $275 pp. On airfare watchdog, there's a promo for $159 pp, but you have to fly by Jan 9; we're so close (our fly date is Jan 17)! But, I am hopeful that we'll get a good rate eventually!