A lot of crypto holders don't celebrate Christmas (especially from Asia and the Middle East) and crypto is a pretty shitty gift unless the person already is into it. I'd say less than 0,5% of >5 ETH investors will gift ETH for Christmas and they will gift $25 (?) on average. Some of the people who get ETH will sell it, some will buy and some will just keep it. Overall I don't think the total amount gifted is enough to affect the market. I think the long term effect is slightly positive because network effects.
A lot more people will get cash and/or return some of their gifts and use the moneyto buy crypto. If anything, I'd say Christmas is slightly positive for the short term price.