What recurring environmental cycle did Pokémon Black and White introduce?

Pokémon Black and White introduced a seasonal cycle.

The games feature spring, summer, autumn, and winter, with the season advancing every month rather than matching the real-world calendar. Seasonal changes alter the appearance of locations, including foliage and snow, and can affect which Pokémon appear in certain areas.

Deerling and Sawsbuck are especially visible examples of the system because their forms change to match the current season. Some areas also become accessible or behave differently depending on the season. This gave the Unova region a stronger sense of passage and environmental variation than earlier Pokémon games.

The seasonal cycle is distinct from the day-and-night cycle introduced in Pokémon Gold and Silver. Weather effects existed in earlier games too, but weather is a battle and area condition rather than the recurring four-season system added in Black and White. The games also introduced Triple Battles and Rotation Battles as separate battle mechanics.