It happens on Saturday of week 2, and requires you to have gone to the forest originally with Allen/Henry/both of them, and that one of them wore the panties.
At some point in week 2, you'll be invited to the forest by one/both of them. If you went with just Allen and "nicely" convinced him to wear them, then you have to compliment him to Miss Dessel first.
When you're in the forest on Saturday of week 2, it's either forced or obvious what choice you need to make to get tied up.