Eevee will evolve into Espeon at max happiness. However since FireRed/Leaf Green have no internal time you will need to trade to Emerald. (Since by now Ruby and Sapphire's internal clocks are no longer working and day/night doesn't happen anymore) and I've not tested Emerald's. In order to get an espeon you have to have its friendship in the evening. To evolve eevee to espeon certain conditions must be met. It must have it's happiness all the way up. It mus be between 12pm and 12am. In Pokemon XD, one could use a sun shard to evolve eevee into espeon.
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When you give it happiness (on heartgold) go check in Goldenrod City the happiness. If it says-"I think your Pokemon really trust you" level it up and it will evolve.
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if i get eevee in level 20 what level eevee need to evolve into espeon
Evolve it from eevee
You can't make eevee evolve into espeon by leveling it up. You have to give it a certain stone. I think it might be a moonstone, but maybe that makes eevee evolve into umbreon.
in the morning, after 8:00, you evolve eevee into espeon, at night, after 8:00, it evoles into umbreon (make sure eevee likes you alot)
There is no stone needed to evolve (although i think it should evolve with sun stone) .But your friendship with the eevee has to be very good then and only then will it evolve into an espeon