To activate this event, you will need to have the following requirements met with certain characters by Year 2, 7/1:
- Cordelia - Friendship 30+
- Lionela - Friendship 20+
- Pamela recruited from the Catacombs and has opened her shop
If you have the above requirements met, an event will occur on Year 2, 7/8 in which Rorona and friends will make a trip to the nearby lake, unlocking the trophy along the way. Note that the trip will last for 10 days (7/8 - 7/18), so plan ahead if you have outstanding requests.
If you followed my advice in following Krystal109's walkthrough, you'll earn this naturally.