Are you using your coins to buy Xmas gifts and to pay for your holiday expenses?
First of all, Christmas is the same every year, but I don't think bitcoin is falling in December each year. It did last year, sort of, and we can say that it does this year, but what about, say, 2017?
As for personal usage, I did sell some BTC to buy presents for New Year and I also received my New Year present in Bitcoin in advance. So some movement can be explained by actions like that, but I don't think it's likely that this is the main reason the price if going down.
Moreover, this negative trend can be over tomorrow or in a couple of days easily, that's just how volatile bitcoin can be.
The price of the bitcoin is not depends on what kind of months afterall. Even if it is december, we cannot say that the bitcoin will become bullish. Right now even it is December the trend is still downtrend. My goal is to increase the bitcoins that I have today so I do not plan to use it in order for me to buy christmas gifts. It is still better if I will use fiat money n buying goods and services to not decrease the bitcoin that I have.