From Bulbapedia, the community-driven Pokémon encyclopedia.
The Luminous Spring (Japanese: ひかりのいずみ) is the only place in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness where players can evolve Pokémon. It is considered the Mystery Dungeon 2 counterpart of Luminous Cave. Luminous Spring is unlocked after defeating the guild members in Mystifying Forest. During much of the game, Luminous Spring could not evolve Pokémon due to time being distorted. However, after graduating from Wigglytuff's Guild, Teddiursa, Ursaring, the player, and their partner, rediscover it, and Teddiursa evolves into an Ursaring. The partner tries to evolve too. However, the player cannot evolve the starter or partner until Manaphy has told them about Marine Resort.
After defeating the guild members, Luminous Spring is not accessed through Mystifying Forest, but can be accessed directly through the location menu.
Pokémon that evolve at a certain time of day (e.g. Riolu) will need either the Sun Ribbon or the Lunar Ribbon depending on the time of day they evolve. Other Pokémon just need the required level and/or items.