From Bulbapedia, the community-driven Pokémon encyclopedia.
Celadon Mansion is found in Celadon City, so hence the name. This is where Red, Blue and Yellow players can get their diploma, when they have caught all 150 or 151 Pokémons. This is also where the makers of the game put theirselves in the game. It is also similar to the motel in Lilycove City in Ruby/Sapphire/Emerald.