I'm not really sure that there is a time of the year that is best to purchase your airfare. Here are few suggestions. Check with online sites like orbitz, expedia, priceline, etc. These are all online discounters for airfare. Spend some time researching prices before you purchase and check all the online discounters. Also check directly through the airlines, I have found many situations where they are lower than the online discounters. Also watch for advertisements online, in the newspaper, etc. Sometimes they will offer special rates for a certain day, I find this quite often happens.
The best way to make sure you get the lowest price is to check every day for one week. The prices will change each day and this will give you an idea of what day your airline will lower prices. Some do it Monday, others Wednesday and some on both Tuesday and Thursday. So it will take some time, but this will help you get the lowest rate.
Also, be very flexible. If you are willing to spend more time flying and having more layovers, you can get a less expensive flight. You may have 3 or 4 layovers, but if you don't mind getting in a little later it will save you a lot of money.
I just bought tickets for California in September and I think I paid about $250 dollars per person, with all taxes and fees. This isn't too bad and it was a pretty good flight, I'm not sure if this is in your budget or not. But I found that the ticket was ten dollars less booking through American than the online discounters.
Also, certain companies will take different discounts. You may find a discount using a AAA card, AARP, teacher discounts, nurse discounts, Sams Club, etc. You can call the companies to see if they take any of these discounts and sometimes they do. Sometimes you won't even think of a discount they may offer that they have, it just depends what they accept.
You may also want to check with your local AAA office. They may have a lower rate. The only problem is that they add a fee on to the air because they don't receive commission on the air. I believe they charge $50 for members and a $100 for non-members (I'm not certain on the actual fee, but I think this is close). So if they have a really good deal on the air you may be better off.
I hope this helps. Good luck with getting your air and have a great trip!