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.
It should be going that way but what had happened is the opposite, the price of bitcoin has a higher chance to increase rather than decrease, it has been proven in 2017 and last year in 2018 bitcoin price rise from $3200 on December 17th to $4000 by Christmas, don't know why. Hopefully by the end of this this year bitcoin will rise again to $10K.
December 22nd 2016 = $820 > December 27th 2016 = $910
December 22nd 2017 = $15,500 > December 27th 2017 = $16,839