Why do in most ghost stories, ghosts appears at night or midnight?

Ghosts in ghost stories commonly appear at night for several reasons:

Because we're naturally predisposed to be scared of the dark. During the day time, we can see predators around; we can't during the night time. We are more vulnerable.

1. Because we can't see well in the dark, and thus it's easier to imagine things to be there that aren't. Our brain can assemble different pieces of information to create an experience that didn't actually happen. For example, we can hear things at night but we can't see them.
2. Because nighttime is dangerous and bad, and thus of course evil things should come out at night. It's a (semi-)logical conclusion that ghosts should prefer the night over the day.
3. Because we have a lot of experience with not seeing ghosts during the daytime, and considerably less at night. That is, if someone were to tell you that X tends to happen at 4pm, you'd say "no, it doesn't." If someone were to tell you that X tends to happen at 4am, you'd be a little less sure. You aren't awake all that often at certain times of night.

Our heightened state of fear, our inability to accurately perceive stimuli at night, our assumption that evil things should prefer the dark, and our comparative lack of experience with the night makes us see "ghosts" that aren't there.