People buy a lot of bitcoins during those holiday times.
I think people are actually selling more than buying at this time of year, at least those who celebrate Christmas and New Year. Most will actually spend some
BTC (by selling for fiat/or directly) to buy gifts, food and drink, decorations and all that is being bought this time of year.
People actually buy BTC long before so they can sell it and make a profit in this period, and this was a good strategy this year as well, at least for those who bought in the first few months of this year, they doubled their money.
Dont forget its a 2 way thing, I wanted to argue as well but I thought If there are no sellers there wont be buyers so you are both right on higher trade at this time. The motive at this time may just be different, some are to take profit while some are to accumulate for the next bull. Buyers at this time are just smarter than sellers.