TL;DR:
Adds seasonal "affixes", making every season unique in its own way.

Whenever a new season starts (excluding Year 1 Spring), players will be presented with a choice - which affixes should be active during the whole season. Those affixes try to make the gameplay more interesting, skewing from one playstyle to another. Affixes can be positive, negative or double-edged/neutral. Some are mutually exclusive.

A lot of the aspects of the mod can be configured via Generic Mod Config Menu.

The mod should be compatible with multiplayer, split-screen and gamepads. If you find any issues regarding these, let me know.

The mod also includes a powerful API for other mods that can be used to control the currently active affixes or add new ones.

There are also some console commands, making it possible to test/tinker with the affixes, or add the mod at anytime without having to wait for another season:
affixes_list_all, affixes_list_active, affixes_list_seasonal, affixes_list_permanent, affixes_activate, affixes_deactivate, affixes_deactivate_all, affixes_queue_choice and affixes_clear_history.
Type help command_name in the console to see more info.

Current affixes

<Vanilla Skill> (one for each vanilla skill)
Increased <Vanilla Skill> level by 3. <Vanilla Skill> experience gain increased by 20%.

<SpaceCore Skill> (one for each SpaceCore skill)
<SpaceCore Skill> experience gain increased by 25%.

Accumulation
Cutting trees has a 30% chance of dropping their Tapper product.

Agriculture
Unprocessed crops are worth 200%.

Boons
Cutting down trees spawns forage underneath.

Bursting
Monsters leave behind an ignited bomb after death.

Caverns
Some mine floors are now filled with gems.

Competition
Arcade games grant money. Qi Coins can be exchanged back to money.

Crows
Scarecrows do not work.

Descent
The last monster on each floor will always drop a ladder.

Drought
It never rains.

Fairy Tales
An extra 15% chance for a Fairy or Witch nightly event.

Hard Water
Sprinklers do not work.

Hivemind
Bee Houses produce faster depending on how many flowers there are around them.

Hurricane
Natural forage items do not spawn.

Inflation
Most gold prices are increased by 20%.

Innovation
All machines take 25% shorter to process items.

Loot
Increased loot from monsters.

Love
2x friendship gain with romanceable characters.

Meteorites
An extra 15% chance for a Meteorite nightly event.

Migration
Fish from any season can be caught.

Mud
Sprinklers work in the sand.

Overgrowth
Harvesting crops spawns forage underneath.

Poor Yields
Crops do not regrow.

Ranching
Unprocessed animal products are worth 200%.

Regrowth
All crops regrow.

Resilience
Monsters have 2x as much health as usual.

Rust
All machines take 50% longer to process items.

Sap
Tappers only produce Sap.

Seafood
Unprocessed fish and roe are worth 200%.

Silence
You cannot speak.

Slumber
Wake up 4 hours later.

Tenacity
Fish are 1.5x as hard to reel in.

Thunder
Storms are 2x as likely as usual.

Tides
Common fish become uncommon.

Treasures
Artifact spots have 25% chance of containing Artifact Troves.

Wild Growth
Trees on the farm grow rapidly.

Other info
You can find the source code here.