Every character you create has a full set of Favorites and Traits. Traits being things you're born with and don't change easily, everything else is a Favorite. One of these is your character's favorite Season. Unlike your favorite day, which the same bonus on your preferred day of the week, every season has it's own individual bonus (and penalty). Just for fun, each Season choice also correlates to a Steam Achievement, so having a diverse set of characters associated with your Steam account can be beneficial.
Springtime, when the scent of life is in the air. When it stops getting cold and the sun finally comes up at a reasonable time of day. Characters who love the spring are cheerful people who are always looking on the bright side of things. They love fresh air and to remind those around them when life is worth appreciating. This positive outlook provides a universal Happiness bonus to whatever District they choose to live in. Unfortunately, the free-spirit attitude leads to disorganization. Characters who favor the Spring suffer a minor Storage penalty. Their opposite is the Fall, though this currently has no active effect.
Summer is the time of relaxation. When even the most weather-dependent work is done and it's not yet time for Harvest. Characters who love the summer enjoy long, lazy afternoons and evenings that go on forever. This is the time for starting and finishing projects, and for waging vicious but somehow friendly warfare. Characters who love the summer pay slightly less to upkeep their personal troops. However, this love of long free days leads to day-dreaming and so these characters also suffer a minor Processing penalty.
Favoring the Fall means you love watching it all go down. From the Harvest through Halloween and then on toward the Holidays, there's never a dull moment in the Fall. Even for those whose lives aren't ruled by the fields, there's always plenty to do in preparation for the cold winters. Food to store, blankets and cloths to craft and Holiday Sales to consider. Fall loving characters benefit from a harvesting bonus, but somehow always manage to slightly overpay their recruits.
Characters who love the winter like to stay busy during the cold days and long nights of winter. There's always a task to do and maybe a story to tell. Whether social or loners, these people tend to be quiet and industrious. They enjoy getting work done before the sun goes down, then settling in for more relaxing work next to the fire. Characters who prefer the winter benefit from a Crafting speed bonus. On the other hand, they don't do well in the heat of the sun and Harvesting is more difficult for them. Winter opposes Summer but there is currently no effect for this.
Characters are allowed to love no particular season over any other. For making this choice, they suffer no seasonal penalty, but they also gain no bonus.