Which main Pokémon game pair introduced the day-and-night cycle?

The main Pokémon game pair that introduced the day-and-night cycle was Pokémon Gold and Silver.

Released in Japan in 1999 for the Game Boy Color, Gold and Silver added a real-time clock that tracked the current time and day of the week. This made the world feel more dynamic than the original Pokémon Red and Blue, where encounters and events did not follow a daily schedule.

The clock affected which Pokémon appeared and when certain events occurred. Some species were associated with morning, daytime, or nighttime, encouraging players to revisit routes at different hours. The games also introduced a day-of-the-week system for recurring events, giving the Johto region a stronger sense of routine.

Diamond and Pearl are a common source of confusion because they expanded the system into five periods—morning, day, afternoon, evening, and night—but they did not introduce it. Gold and Silver also debuted other major series features, including Pokémon breeding, held items, two new battle types, and 100 new Pokémon.