One word "retail".

Working in retail I can vouch first hand for how Christmas is the biggest sales opportunity of the year - it's when retailers use all their tactics (subtle and not so subtle) that this is an important event and you will fail at it unless you buy more stuff.

Except that buying more stuff doesn't really make people happier. As long as we have enough that is all we need. If you are Christian that is the message Christ gave.

Then comes January and the inevitable purge, people have eaten too much, spent too much and so that is a very lean month.