it depends
are you trying to catch fish you havent caught before like rare ones bc i would say look up the times theyre available throughout a specific month (here's a website to find when certain things are available http://www.thonky.com/animal-crossing-new-leaf/list-of-bugs-fish-deep-sea)
but if you're just trying to make a lot of bells i would say anytime past 7pm when the sun is down is a good time to catch expensive bugs and fish (especially on the island)
it depends
are you trying to catch fish you havent caught before like rare ones bc i would say look up the times theyre available throughout a specific month (here's a website to find when certain things are available http://www.thonky.com/animal-crossing-new-leaf/list-of-bugs-fish-deep-sea)
but if you're just trying to make a lot of bells i would say anytime past 7pm when the sun is down is a good time to catch expensive bugs and fish (especially on the island)