Both AC and WJ generally open their bookings at 300 days (10 months), US airlines don't generally have as much lead time (making comparison shopping very difficult for the early bird). Tickets usually open at an "average" price and then the computers take over using yield management to adjust the prices (they don't have people changing the prices in the background anymore). Thus you will notice that prices can fluctuate during the day as demand changes.
So what happens for popular flights at busy periods (i.e. March break for Orlando) is that the prices can often start to rise dramatically and quickly as the sales are faster than average. For slow periods, the opposite is true; if there are not enough takers at the higher price, the cost starts to slide downward until they are almost giving them away the week before departure.
So all you really need to know when booking is how popular your flight will be. If you expect not very popular, wait around and shop around. If you expect it is popular, buy early and watch the prices rise for all those other people who are waiting around for a bargain. Also note that more travellers head to vacation destinations on the weekend, so taking a weekday flight can save you money; the opposite is generally true for business destinations like Chicago or Philly.
