Yuko Magaki’s painting depicting the Day of Tranquility. Photo credit: [1]
The Day of Tranquility were two separate occurrences of disaster striking Rogetsu Isle in Fatal Frame IV: Mask of the Lunar Eclipse. The first occurrence was on an unrecorded date while the second occurred roughly ten years before the events of the game.
First Day of Tranquility[]
Before the first disaster, Yomotsuki family head Soetsu, created the Mask of the Lunar Eclipse using forbidden techniques to purposefully corrupt the Rite of Descent. Soetsu's goal was to open a portal to the Hallowed Lands so he could explore the afterlife. This plan ended in catastrophe. The Vessel wearing the Mask of the Lunar Eclipse lost herself when the mask erased her soul, becoming a void and instantly Blooming. All who saw her immediately and peacefully died, as their souls passed through her to the Hallowed Lands upon seeing her Bloomed face. The calm manner of their death is why the disaster was called the "Day of Tranquility".
The Vessel then proceeded to roam the island, drawing all who saw her into death. Many islanders took shelter hiding in sacred underground tunnels in an attempt to hide from the wandering Vessel.
The islands Guardians were able to stop the spread by playing their Moonsong from the lighthouse.
Those who survived considered the ritual as taboo, and forbade anyone from repeating it, destroying many sacred texts and practices to ensure it was lost forever. Soetsu mysteriously disappeared after the crisis, fate unknown, and was never seen again.
Second Day of Tranquility[]
In the 1970’s, Shigeto Haibara, head of one of the most prominent families on the island and the director of the Haibara Hospital, became obsessed with recreating the original Rite of Descent as a way of curing his daughter of Moonight Syndrome and finally controlling the disease. Shigeto and his son used a Camera Obscura to resurrect many of the destroyed sacred texts lost to time.
He approached the Yomotsuki family head, Soya, to recreate the Mask of the Lunar Eclipse for his attempt at the Rite of Descent. Soya became obsessed with the project, his notes saying he was trying to “be” Soetsu and surpass him where he failed. His obsession caused him to drive away his wife and child.
With the mask completed, it was given to Shigeto who used it in his Rite of Descent using his daughter Sakuya as the main Vessel, along with five kidnapped girls from his hospital to act as Organs. This Rite took place in the Temple of Tsukiyomi, underneath the concurrently-performed Rogetsu Kagura being performed on the stage above it on the surface.
The Rite was a failure, likely for many combined reasons. Sakuya and the girls all suffered from Moonlight Syndrome, corrupting their Moonsongs, and likely underwent insufficient training prior to the ritual. It's highly possibly some sacred texts or practices remained undiscovered, and the guidance of the Guardians was not available. The Temple of Tsukiyomi had been sealed after the forbidding of the original Rite, and it makes sense, given the nature of the Rite and it's relationship to the Moon, if it originally had a sky light or similar which was not re-opened due to the existence of the Kagura Stage built on top of it. The recreated Mask of the Lunar Eclipse shattered during the Rite. The moment it shattered, Sakuya and the kidnapped girls collapsed into a comatose state, while the performers involved in the Rogetsu Kagura above all died of heart attacks.
While the five girls were later found and recovered, (albeit with amnesia) Sakuya remained in a coma and was placed underground while she remained asleep.
Two years after the failed Rite, Sakuya awoke from her coma (the trigger for her awakening is unknown). Her face completely bloomed and she broke out of her confinement, wandering the island and turning everyone who looked at her. This resulted in the second Day of Tranquility.
Soya, fully aware of the situation, awaited his death in the underground caverns and contemplated what went wrong. He realized the Rite was missing the crucial Moonsong, an ancient song passed down by the Guardians of the island which was said to soothe the soul. Without the Moonsong or the Guardians to stop the fully Bloomed Sakura, every person present on the island died during the Second Day of Tranquility. The bodies were discovered when a ferry boat landed, and news of the bizarre, unexplained, mass death of the islanders was reported in many newspapers.
After this second disaster, an investigation was placed to find clues as to what happened to no avail. The island was left completely abandoned.