Instructions
Raise your Eevee's happiness in "Pokemon Sapphire." Keep it in your party, give it Pokemon Vitamins and use it to battle often. Go to Verdanturf Town and enter the home next to Wanda's house. Speak to the woman inside. Increase your Eevee's happiness until she says, "It adores you. It can't possibly love you any more. I even feel happy seeing it."
Wait until the in-game time of "Pokemon Sapphire" is 12 p.m. to 12 a.m. You set the time at the very beginning of the game and you cannot change it. Return to your house to check the clock in your room. The clock displays the in-game time.
Train your Eevee until it levels up. You can train your Eevee by battling wild Pokemon and Pokemon Trainers. Once it levels up during the day, it will evolve into an Espeon. For example, a Level 5 Eevee will evolve into an Espeon once it becomes Level 6.
Use the GBA Link Cable to hook up two GBAs. One GBA will play "Pokemon FireRed," while the other will play "Pokemon Sapphire." Load your save file and head to the nearest Pokemon Center in both games.
Ascend the escalator in the Pokemon Center in "Pokemon FireRed" and "Pokemon Sapphire." In both games, talk to the female behind the second counter. The woman will ask you to enter a room. Your "Sapphire" character and your "FireRed" character will automatically take a seat. In "Sapphire," offer the Espeon. Offer any Pokemon for the Espeon in "FireRed." Select "Yes" and the trade will commence. You now have an Espeon in "FireRed."