They start at 7PM and only occur on a clear night. You are not guaranteed meteor showers per zodiac cycle or anything, although random shooting stars can still happen.
Certain times of the year have more frequent showers, the companion guide had them listed, but I don't have a link to where I saw it last.
Meteor showers are also date locked per switch. Meaning the days they appear are different for everyone, however once you get it on say July 4, 2020 (which is one of mine) you can time travel back to that date and still make wishes.