How do you do Weekdays Request on HorseIsle?

First you must do Robinson's quest, find Weekday, he'll be right next to Robinson on Vine Isle. He says he'd like to get Robinson a shelter, and he asks for your help. Go north to the rock shop and talk to the owner. He'll tell you about an empty tree house, return to Weekday and tell him you've found a place for them to live.