Another Air Tran traveller here. I believe that as threeprincesses guesses, they only sell a certain number of seats at the lowest price. For my flights, the price then jumps up $30/ticket when that number of seats is gone. We had originally made reservations for June 4-11 (Fri to Fri) but hadn't bought air fare. I was watching the Air Tran site and about lost my lunch when the fare suddenly went up $30. No other days/flights had gone up, so I had to guess that the cheap seats had sold out. We decided to leave a day earlier (which at the time didn't save us money because we had to add another day to our package). My dh very much wanted to wait until after the first of the year -- he couldn't believe I wanted to buy tickets for June in November. I convinced him we should go ahead and buy them in November. Within a week, the cheap seats for our return flight sold out. So if we had waited, we would have spent $60/round trip x 5 tickets = $300. That's about what we're spending for an extra night/day at Disney. I'd rather have the extra day.
As for sales, Air Tran does run sales to Orlando, but I notice that they are only about 4-6 weeks prior to departure dates and the max discount from my airport is $14/each way. In this case, I'd rather know I have the tickets than wait until a month prior and hope for a sale.