Traveling around Christmas and New Years can be ridiculously expensive, but if you need to visit family, or you want to capitalize on those paid holidays, the best time to book is in the summer.
If you're flying from a smaller airport, definitely use Priceline. You can get a ridiculous discount on flights through their Express Deals. For example, Grand Rapids, Michigan to London, England is about $1,400, but Priceline has an Express Deal for $700. The problem with Express Deals is you won't know the airline, time, and layover information until after you book. The only information you'll get is general takeoff time (morning, mid-day, or evening) and maximum layover times. Your ticket is also non-refundable once it's booked. You can't even make a change or cancel your flight within the first 24 hours. So make sure everything is correct before you book an Express Deal.
Finding a good, cheap hotel around the holidays is near impossible. And, unfortunately, less Airbnbs are available during the holidays. For the best holiday deals, I suggest using Priceline Express Deals again. See how to unveil anonymous hotels here.
If you're not quite ready to book, or you missed out on summer savings, wait until November to book your holiday travel. There are often amazing deals throughout the week of Thanksgiving, including Black Friday and Cyber Monday. I also suggest setting up fare alerts on Google Flights or Hopper.