A good reference about weight gain, calories and alcohol at Christmas is Reid & Hackett (1999) J Hum Nutr Diet 12 513-16.
They found an increase in weight of 1.2kg in the subjects who had been eating well over Christmas (excluding 5 who were ill). 15% of the subjects gained more than 2kg. They looked at - a 15 day period was used. It is highly likely that with longer celebration holiday periods as is now the case this weight gain would be more. They put it down to increased food and drink as well as more inactivity.
They also found blood pressure rose significantly in some of the subjects despite it being a supposed tome of relaxation. Stated may be due to increased weight and increased alcohol above 4 units per day.
Another study Rees et al (1985) BMJ 291 1764-1765 showed weight gain and excess intakes of 6000 kcals over a 5 day period which on its own would be expected to cause weight gain of > 1kg week - so 2kg over the 2 week period is highly likely.