Which pair of Pokémon games first introduced the series’ day-and-night system?

Pokémon Gold and Silver first introduced the Pokémon series’ day-and-night system.

The second-generation games used the Game Boy Color’s clock and tracked different times of day, including morning, day, and night. This affected encounters, events, and the appearance or availability of certain Pokémon.

The system added a sense of time passing to the Pokémon journey. For example, some species were easier to encounter at particular times, while other events depended on when the player visited a location. It also helped make the world feel different across repeated visits.

The feature is sometimes associated with Pokémon Crystal because Crystal expanded and refined several elements of Gold and Silver. However, the day-and-night mechanic debuted in the paired Gold and Silver releases in Japan in 1999. Later games continued to adapt the idea with more detailed clocks and schedules.