If you want to know when you can see Wisp, it's going to have to be after 10 PM, and it'll be random as to whether or not he shows up. There's no guarantee that he's going to come every night, so you're going to want to keep your eyes peeled and to be wandering around at exactly the right time.

How often does wisp appear in Animal Crossing?

In Animal Crossingedit

In order for Wisp to appear, the town must have 8 or more weeds. Once this requirement is met, Wisp will appear on a random night in 2 to 4 days.

How often do you get wisp?

Wisp can be found floating around the woods sometime after 10pm occasionally. He will stay in your town until the start of the next day, which begins at 5am.