Question: what is the best time to evolve eevee into umbreon?
Answer: Walk with your Eevee Buddy for at least 10 KM and until you get two at least Eevee candy. (You need to have gotten two candy for this to work.) While that Eevee is still your Buddy, hit the Evolve button during the day (game screen in bright) to get Espeon, or at night (game screen is dark) to get Umbreon.