Specifics of 'Evolve Eevee into Umbreon at night' - 10KM, 2 candy, at night?

If you only want to evolve Umbreon for the Pokédex entry, the easiest way is to rename your Eevee to Tamao and it will 100% evolve into Umbreon without needing any of the other requirements. Note that this only works once. This is possible once-per Eeveelution, see this answer for all possible names that work.

Here are also a couple of other nuggets that will be useful if you want to get an Umbreon the normal way: https://www.reddit.com/r/TheSilphRoad/comments/96o6nu/psa_the_umbreonespeon_method_is_walk_10k_for_two/

• "Night" is when the in-game sky is dark, and in the case of sunny or partly cloudy weather, the weather icon shows a moon instead of a sun. Basically when the game itself goes into "night mode".
• In addition to the OP's message, you must have a valid GPS location when you do the evolve, as this is needed to determine whether it is day or night at your location.

  Note: Basically do not do the evolution if your GPS location is spotty, like if you're underground in a subway or something.

• You can walk for 2 candies and change buddies but you must make the Eevee your buddy again to evolve.

But personally, if you want Umbreon for Pokédex filling purposes, just do the name method. I did this myself a few years back and it worked, but as I said, it only works once.