December is one month where many different cultural holidays overlap. Even those without strong religious beliefs often have certain activities they associate with the holiday season. That makes it a particularly appropriate time to talk about traditions and worldbuilding.
As far as I know, every single culture in existence has traditions that it relies on (they are generally different from culture to culture, but every culture has some). Whether they started for practical reasons, superstition, or simple happenstance, traditions set rules that society reinforces and that can be exceedingly difficult to change. That makes traditions vital to worldbuilding.
First off, not having traditions is liable to make the world seem a bit flat or unrealistic. Like ignoring the everyday stuff, forgetting traditions can weaken your worldbuilding.
It also misses a major opportunity.
Don’t forget that traditions set rules. That makes them absolutely fabulous for building characterization as well as complicating and driving the plot. A tradition that doesn’t fit with what the character wants to do establishes instant conflict. Feeling obligated to uphold a tradition despite difficult situations can cause a character to do something foolhardy or reckless. Missing traditions or adjusting to the strange traditions of a different culture – there are so many ways for the world’s traditions to tie the characterization, plot, and worldbuilding together (talk about useful tools!).
Even something as simple as celebrating a nationwide holiday can have a big influence on the story. Think about how it changes our own lives: travel, shopping, decorations, music, extra religious ceremonies, and stores being closed (to name just a few).
How will holidays affect your world?