@Sharkbiscuit if you switch to weekly chart and look at December for any of the major coins such as BTC, ETH, LTC, etc. you'll see they all pumped during the entire month of Dec. It was rockets everywhere. A truly epic run.
I believe the majority of the tax-season selling happens AFTER new years.
1. Those who have only accumulated during the year and never sold (no cash out) don't have to pay capital gains taxes. This is where waiting until the new year to cash out is beneficial. Delay taxes on those big gains for another year.
2. Leaving your money in longer you get a chance to grow it. People generally feel good around the holidays and sentiment is running high for new years aspirations, financial goals, personal goals, etc. so it makes sense there would be some buying around the holidays.
3. I believe majority of people really don't start thinking seriously about their taxes until sometime in January. Out of site, out of mind sort of thing. Or maybe they just don't want to face reality of cutting a fat check to the gov until they absolutely have to. Everyone I know seems to wait until March or later to get their taxes done.