You should book it when you feel the price is what you can handle paying or what is reasonable.
I go the week following T-giving week and I usually travel alone - I fly Airtran, so I can take advantage of non-stop flights from BWI and using the RAC to return home.
The flights for Airtran are not out yet for that time. Wait until the fares are posted for that time period and then watch the trend for a few weeks or so.
You will see the flights are a bit more expensive for the weekend before Thanksgiving, as they are for me the Saturday after Thanksgiving. But they will still be reasonable.
Here is what I did. The first day the flights are posted, you will only see the highest flight charges posted. The web specials will kick in after a few days.
I noticed that from Tuesday to Thursdays, fares posted were less than from Friday to Monday. I believe that it is the time when families are home and are planning their trips. I started looking early Tuesday/Wednesday/Thursday when I got to work at 6:00 am. The fares seemed really low, then would go up around lunchtime and then go down a bit, but never as low as it did first thing in the morning.
I finally booked mine on a Tuesday morning at 6:00 am, when it got to $170.00 round trip - I thought it is never going to go down anymore - and it didn't. I flew the Saturday after Thanksgiving and flew home the following Saturday, on the exact two flights I wanted.
It is like russian roulette, once you figure out the odds and how well you are going to come out - just get it over with and book it....