The background climate, and the source every other module draws from.
The vision
Season is not out to decorate the year: it installs a background climate beneath the world that everything else will consult. A configurable calendar paces the cycle, a season lasting two in-game weeks by default, and provides the base climate variables, temperature first.
Those variables are not decorative either. They decide rain and snow, what grows, what migrates, what bites the hook. Season is the suite's source of truth for climate.
Climate does not follow a north-south gradient, which would force you to always walk the same way to change airs. It is organized as concentric anomalies around the spawn point: hot, temperate and cold circles repeating as you move away.
Interdependence rules keep the whole thing believable: hot zones stay dry and never see snow, cold zones stack snow and rain, and the intermediate temperate zones get the most rainfall.
A desert does not turn polar in winter. The extremes barely move from season to season: it is the temperate zones that truly swing between hot and cold, and so they are the ones that live the seasons as we mean them.
The move from one season to the next happens by continuous drift, interpolating day after day between profiles, rather than by an abrupt switch. Temperature slides instead of jumping, in the same spirit as a day's fade.
Since the climate is concentric, the phase of the seasons can be offset by distance to the spawn point: the temperate ring lives its summer while the neighboring ring enters autumn. Seasons then travel through the world instead of landing everywhere at once.
That gives a real reason to move, and incidentally provides a natural engine for animal migrations: the herd does not follow a script, it follows the climate.
No season bar shows on screen. The season is read from the world, and if an object must state it, that object is craftable and held in hand: an almanac showing the current season and the next.
The year can also hold its exceptions. A heat wave or a deep freeze, rare and telegraphed in advance, put every climate-dependent module under strain. A spike you can plan for beats a forever-lukewarm average.
Season publishes, the others react. Weather derives its simulation from it, Water the fate of the rain, Biome the density of its flora, Farms the yield of its crops, Animals its migrations, Fishing what bites. Climate stops being scenery and becomes the world's prime mover.