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.
Indeed, also if OP based it on what he observed last year, it is probably because the majority of us bought bitcoin last December thinking that in this year, bitcoin will skyrocket again and make another All Time High price but it didn't, and it could be repeated over and over again because we have our own predictions about bitcoin's bull run, moreover, it isn't that bad to buy bitcoin in this so called "Bitcoin week" by OP since bitcoin is still in the deep.