Which jobs are dependent on seasons and how?

Solution 1:

Well like you said, Farmer is 1 of them. They will only do their farming from early spring to early autumn. Besides that, traders only do their job when a merchant is present. I personally keep the farmers on their job at all times. And take the traders off their job once there is no merchant in sight.

Finding the right balance for your farmers/traders in the "off-season" is really dependent on where you need them the most. If you need food, give them a gatherer job. If you need buildings send them off building.

Solution 2:

One aspect not mentioned yet is that during the winter, any citizens which do not have indoor-jobs will visit warm houses for warmth from time to time. This affects the productivity of running-around jobs like hunters, gatherers, herbalists or foresters. It also greatly affects any resource-removal operations which take place far away from your town.

Solution 3:

Farmers actually need to spend very little time in the crop field:

• They start planting in the spring, and plant up until the 'Harvest' button becomes click-able.
• At this point, the field is fully planted, and they 'tend' to the field. This is completely unnecessary, and does not affect the yield of the field - they can be manually assigned to labourers in this period.
• Once the yield reaches 100%, farmers are then required to start harvesting the crop.
• Once this is finished, they are free until spring (and automatically work as labourers).

Farmers working in an orchard are free between late autumn and mid spring.

Hunters have a 2 month wait after killing a deer.

Other professions act as labourers when their product has reached your limits, there is no space for their product, or when there are no raw materials.