Wisp comes on random nights around 8pm-1am. If he is in your town, you'll see an empty lamp near a building or a house before the time mentioned above. Then when you walk around randomly he'll speak to you.
I personally have not seen Wisp myself so I cannot confirm this information. This was from Let's Go to Wiki.
I've also heard that he appears Monday nights from 8pm-1am. If you look for him during that time, you should also find him.
These haven't been confirmed by me personally, but they are other options to consider. =3
Both times Wisp has come to my town I have found the lamp during the day just randomly. Then I know to go to the beach and walk around at night. You won't see him, but when you get close to him, a speech bubble will pop up and he'll tell you which way to walk to find him. Then he'll reveal himself and tell you to go to your attic.