If you are traveling at holiday time, you should book your flights as soon as they are released. I normally travel over holidays and try to be at my computer on the day that schedules open up, first thing in th morning. I have found that even delaying for a couple hours results in a higher price.
I hope you aren't asking about this Christmas. If you mean to travel this Christmas you are already way passed when you should have booked. Now its just luck of the draw to see if anything comes down. My Mom once booked late and paid $800 rt but then it dropped to $600 so she got a credit. This is from Boston where fares normally run around $200-250 when it isn't a holiday. You will not get a cheap fare over Christmas holiday.