The different seasons in Assassin's Creed Shadows offer different weather effects - but you shouldn't worry about them affecting your gameplay.
In an interview with Play magazine, Assassin's Creed Shadows art director Thierry Dansereau explains that Ubisoft didn't want the changing of the seasons "to be something that could be perceived as very negative or have a negative impact on how you play." Instead, any changes caused by the seasons will likely be limited to the animation level.
