Eevee evolves into Umbreon by happiness at night. To evolve make Eevee happy give it EV increasing medicine, Poffins(In Sinnoh games), massages, and the soothe-bell. Do lots of battles but do not let it faint. Then level up between 8 pm and 6 am but be careful not to level up during the day or else it will evolve into Espeon.