Travis Saunders at the Obesity Panacea blog notes that average weight gain in adults over the Thanksgiving (U.S.)–Christmas–New Years’ season seems to be on the order of 0.8 pounds or 0.37 kg.
Data are from a 2000 article in the New England Journal of Medicine. Researchers weighed 195 Americans throughout the year. My quick search at PubMed.gov found no better or more recent studies.
I mention this because I had written somewhere that average holiday season weight gain is about five pounds (2.3 kg). I stand corrected.