If you're unable to see your child until Christmas Eve, don't abandon your past tradition. Tweak it and make a new tradition out of the Christmas Eve tree-buying process. Take the tree home and decorate it that night. Modify the tradition even further for a new bonus family by making hot chocolate, playing a new favorite Christmas CD or exchanging special ornaments that all of you can admire and talk about in the years to come.
Although integrating past traditions is important to the success of holiday blending, don't forget to establish brand-new bonus-family traditions as well. Make them special and unique to the new family, especially if there are new "our" children added to the mix. Establishing new family traditions demonstrates respect for "our" family members and reinforces the new bonus-family identity.
Finally, try not to force combined family traditions into one particular day, because you may not be spending that day together. An example: "We only open our presents on Christmas morning, no exceptions!" Or: "We eat Thanksgiving dinner at 3pm every year!" This just adds more stress to an already stressful situation. Instead, be flexible and try to create bonus-family traditions that focus on the time of year rather than on that one day, and your cherished times together will become about the holidays or Christmastime, not necessarily Thanksgiving or Christmas Day. That will take the pressure off everyone.