During the Night Market festival in Stardew Valley you can go watch the Mermaid Show, which plays a funky little cutscene. While just watching the show doesn't get you anything besides the short cutscene, there is a hidden secret you can find, if you know the correct password. Here's what you need to know.
The Mermaid Show is part of the Night Market festival, which takes place from Winter 15 to 17. The Night Market is located on the pier, right outside Willy's shop and runs from 5 PM to 2 AM all three days, although the Mermaid Show closes at 12:30 AM. The Mermaid Show is located on the eastern part of the pier.
You can unlock the secret reward by pressing the shells in the correct order after the show ends.
When you enter the Mermaid Show, it will automatically start playing, and it will restart if you leave and come back in. The show takes 1 hour and 40 minutes of in-game time to complete. Once the show ends, the curtains will close, leaving you in the room with 5 interactable shells in a line in front of you. Interacting with the shells will play a musical note, and the puzzle requires you to play them in the correct order.
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The solution is 1-5-4-2-3, with the leftmost shell being 1 and the rightmost being 5. The way you learn this information is by finding the Secret Note with the answer. You can unlock Secret Notes by completing the Winter Mystery quest, which can be done as early as Winter 1. You don't have to have the note to put in the correct solution.
Your reward for solving the puzzle is a Pearl, which you can only get from the Mermaid Show once per save file. Pearls are a treasure that can be sold for 2,500G, but it is also a universally loved gift, if you don't need the money.
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The Mermaid Show
The Mermaid Show is part of the Night Market festival, which takes place from Winter 15 to 17. The Night Market is located on the pier, right outside Willy's shop and runs from 5 PM to 2 AM all three days, although the Mermaid Show closes at 12:30 AM. The Mermaid Show is located on the eastern part of the pier.
You can unlock the secret reward by pressing the shells in the correct order after the show ends.
When you enter the Mermaid Show, it will automatically start playing, and it will restart if you leave and come back in. The show takes 1 hour and 40 minutes of in-game time to complete. Once the show ends, the curtains will close, leaving you in the room with 5 interactable shells in a line in front of you. Interacting with the shells will play a musical note, and the puzzle requires you to play them in the correct order.
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The solution is 1-5-4-2-3, with the leftmost shell being 1 and the rightmost being 5. The way you learn this information is by finding the Secret Note with the answer. You can unlock Secret Notes by completing the Winter Mystery quest, which can be done as early as Winter 1. You don't have to have the note to put in the correct solution.
Your reward for solving the puzzle is a Pearl, which you can only get from the Mermaid Show once per save file. Pearls are a treasure that can be sold for 2,500G, but it is also a universally loved gift, if you don't need the money.
Source