This is just a fact when traveling over the holidays. While there is a chance inventory in a lower fare bucket may be released closer to travel, there is also a chance it will not. Last year my spouse booked a trip over Christmas about 6 months out. It was in high fare classes (H, B, or M). I checked weekly up until departure and never saw anything cheaper. So, once you see a price you are willing to pay... book it. Sure, it may come down, but it might go up.