From Bulbapedia, the community-driven Pokémon encyclopedia.

Celadon Mansion is a large building found in Celadon City. It is here that players of both the Generation I games and their Generation III remakes can find an Eevee, and where players of the Generation II games can find TM03 - Curse, at night.

It is also the home of the Game Freak development studio, where the lead developer will give the player a diploma that signifies that they have completed the National Dex.