Here's the immunogenicity and clinical effectiveness data showing how fast the booster dose works. As you can see you'll start to get some benefit ~1 week after your booster, but full effect will be after 2-4 weeks. So if you are getting the booster in the coming days, I'm afraid it is too late for Christmas.
And some people have asked me how long this boost in protection will last. We don't have much in the way of long-term data yet on the booster. But Pfizer's data shows it is durable through at least 3 months for now.
There is some preliminary data to suggest the booster is driving affinity maturation. We aren't just restoring declining antibody numbers, but potentially improving the antibody response. Prior experience with other vaccines suggest this boost may provoke a more durable immune response than seen with the primary series that may be long-lasting.