I think people refrain from trading or performing task that is related to crptocurrency or even their own work during holidays. The holiday is intended to harness and express the fruit of the labor one have done in the whole year, thus giving more time to self, family, friends, and other. Holiday is also the time of spending so, I think more people will cash out some of their bitcoin to spend in this once in a year event.
I'm really not sure if christmas affects directly to its price decrease but somehow the logic of the circumstance where in many people are selling their bitcoins to exchange to physical has something to do with the supply and demand. However, bitcoin price is volatile so anytime of the year we really can expect a sudden change in its price but we will look forward for increase rather than decrease.
