When does Redd appear?

To add to this, after his first appearance he appears randomly. One person got him 3 days after and another person got him 19 days later.
 
He was just wandering around my island today. Didn't even notice his boat at first.
 
Basically Blathers will mention the possibility of adding art to the museum. The next day Redd appears to give you your first piece to donate and unlock the new exhibit. He'll be wandering around your town the first time so you have to search for him.
I would check how many donations are in your museum if you don't seem to be getting the dialogue from him. It does seem some guides are saying 60 donations are needed in your museum to trigger it like PP said.
 
There's a process to getting Redd and it can overall take 3-4 days for him to appear in his boat. This is the basic process of it.

1. You have to talk to Blathers and he will mention expanding to the art section. Right now, it seems like you need to have 60 items donated between all three sections to qualify.
2. The next day, Isabelle will mention that a shady character is on the island. This is Redd. You need to find him and talk to him. You must buy the art he is selling you and you must donate it. If you don't donate it, the next step won't activate.
3. Once donated, the museum will undergo a renovation the next day.
4. The day the reno is complete, Isabelle will announce it and warn you of fake art. Redd should also be on your island at this point. Once you speak to him, he will allow you access to his boat.

The step I've noticed people missing is donating the first art piece. It's got to be donated to advance the progression for Redd.
 
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