it depnds were u evolve it
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Level up an Eevee with full happiness at nighttime (after 6pm)
Trade it to ruby,sapphire or emerald have them evolve your evee at night then give it back to leafgreen.
No because the game lacks the built in game clock feature which lets the game change from day and nighttime which is essential for evee to evolve to Espeon or Umbreon. If you want these pokemon you will need to use Ruby, Sapphire or Emerald to evolve the Evee because these games have the in game clock feature or you can even trade the Espeon and Umbreon from pokemon colosseum.
Evee will never evolve to espeon or umbreon in firered because the game doesn't have internal time so to get an espeon or umbreon you must trade your evee to a game with a internal clock like ruby, sapphire and emerald. Also you can trade an espeon obtained from colosseum or even xd gale of darkness.
sorry you cant, you have to evolve your evee during the night it will be either a umbreon or a espeon. so its the matter of luck on getting the one you want.