Step 1: Purchase a Red Potion or Blue Potion.
Purchase a Red Potion or Blue Potion from the Magic Hags’ Potion Shop in the Southern Swamp. The shop is open from 6am to 6pm on all 3 days, but it is easiest to purchase a potion on the First Day.
On the Second Day or Third Day, Kotake will leave the potion shop to search for Koume, who is lost in the Woods of Mystery. If you wait until the Second Day or Third Day, you will have to find and help Koume before Kotake will return to the shop and sell you a potion.