The world follows a season cycle that progressively shifts the importance of resources and drives agriculture. Climate is partially dependent on the season, as is weather, though both have a random aspect. There are 12 seasons: Early Spring, Spring, Late Spring, Early Summer, Summer, Late Summer, Early Autumn, Autumn, Late Autumn, Early Winter, Winter, and Late Winter. A new year starts when Late Winter ends and Early Spring begins. Citizens appear to age every 2.4 seasons or 5 times a year. 
During summer, especially later in summer, the amount of stored food will be at its lowest and citizens face starvation.
Autumn should be the easiest season. Crops will be harvested making starvation unlikely and it is still warm enough to avoid hypothermia. However late in Autumn there may be an early frost which will kill any unharvested crops and lengthen winter.
Winter is considered to be the harshest season in the game, because of the low temperature and inability to farm. Food supplies tend to decrease, and your citizens could freeze to death during the frigid months. Strong stores and production of Firewood is a necessity, as is housing, and will help to prevent hypothermia. Warm clothing keeps your citizens warm longer when they are outside their house, but they will still freeze to death without proper housing.