Always decrease? No it's not. Maybe you started your journey in crypto world in a bearish trend that's why your mindset is that. There are lots of factors why the price decrease and rather than to give you a detailed information about that here which will take a long writing, that's your assignment now and find it on research.

And what's special about December? Even the price is always showing a good performance on that month in the past, it doesn't mean we have to considered it as a basis that another price increase will happened that month. No one knows what will happened in the future therefore price movement on December is unknown.