My experience has been that it's best to wait, and track prices carefully. I've been joining the Mensa ScubaSIG dive trip to Cozumel the last 4 years, and going again this year in December. Once I knew the dates (early March), I checked out prices on the ITA App - it was originally $582 RT from DFW. Came down to $542. Then, in early July, I think, I checked it on a Friday morning while having coffee, and it was $382. I jumped on it. By Sunday evening, it was back up to $542, and hasn't dropped below $500 since. I had a similar experience last year, and saved about $120 on a ticket where the sale price was up for less than 36 hours. In 2013, the sale price popped up in late July or early August, so anywhere between 4-6 months out the Airline will figure out that they have too many empty seats on a flight, and run an unadvertised sale to bump up the load factor. Start checking daily -early AM is the best - about 6 months out, and be ready to jump on the price break quickly, because it will disappear as soon as the airline meets its load factor metric.
There is, of course, a risk to this strategy. You might really want a specific set of flights that just happen to book up early, so the airline never gets to a point where it needs to put the tickets on sale to meet its load factor minimum. This risk is less if you have some flexibility - 2-3 days either way. And, since I was traveling in a low demand period - first or second week of December, the lull before the Christmas-New Years rush, I was probably more likely to see such a flash sale. If you are traveling in high demand times, like Christmas-New Year, Spring Break, Long Holiday weekends, the strategy is unlikely to work, and you might well find your desired flights fully booked 6 or even 8 months out.
