I've seen it mentioned in few answers here that ghost type Pokémon appear only at night. Is this true?
No, it's not. For starters, here's a Gastly around 4pm in the afternoon.
I've also been playing the game until 2 am every night since the game was released and haven't seen a single ghost-type Pokémon in the wild at night. It is doubtful that time of day is a factor for them, which means they're probably location dependent, just like other Pokémon.
As far as time of day and the appearance of Ghost (or Fairy)-type Pokémon goes, I definitely wouldn't fault anyone for thinking that there was a relationship there. Multiple sites have created articles claiming as much, and it certainly makes sense that there would be a connection. Unfortunately, based on both my personal experience and evidence posted by other users on this site, there just doesn't appear to be one. I personally have caught two at night and one during the early afternoon, and seen another one on my proximity tracker during the day as well. Ellesedil's answer also provides evidence that Ghost Pokémon are not exclusively found at night.
Unfortunately I have no information about locations where Ghost Pokémon appear. I personally have caught Gastlys near a Park, near a historic landmark, and along a nondescript road near a school, so I can't even offer a guess in this regard. If anyone else has any information (preferably from reputable sources) please feel free to post it to augment our understanding.
Well. Ghost pokemons doesnt only appear at night. Today I walked a pretty far distance looking for pokemons, and suddenly a ghastly popped up right infront of me in the middle of the day. I think you can get pretty much any pokemon at any point of the day regardless of your position. Maybe not water pokemons but.. hehe
My experience is certain types of Pokémon pop up in certain area more than other area.. but not necessarily relates to their genre.. (other wise where to find electric Pokémon?) and no, ghost Pokémon doesn't pops up only at creepy places either..just random to time/location in general depend on their rarity. it is more effective to have poke hunters share where they caught most of what Pokémon in what area, then we all can benefit.