Ryoan (諒闇) (諒闇)
Ryoan is a period when the Emperor is in mourning for his father and mother. It used to be a year long, but was set 13 days by Emperor Ninmyo later. It is also called roan. And when it is written in 諒陰 and 亮陰, it is pronounced 'ryoin'.
It is a period of mourning for the Emperor, the Grand Empress Dowager, and the Empress Dowager. A mourning dress called ryoanfuku is worn at that period. At present, when the Emperor needs to go out as a public service, an announcement to the effect that the Emperor's mourning period is over on the column of the Imperial House things in the official gazette.
In Confucianism, it means a period of mourning for a father should be three years and a mother should be for a year as written in the "Book of Rites". It came from this concept of ryoan that the Emperor's ceremony of the enthronement is now held three years, counting in the old Japanese way, after the late Emperor's death.
In Japan, there is a rule of mourning of the Emperor in the Act of Funeral and the Act of Ceremony and System in the ancient Ritsuryo law; however the period is only 3 days, but usually it is longer. There is a record in "Shoku Nihongi" (Chronicle of Japan Continued) that court functions were canceled when Retired Emperor Shomu's mother, GUJIWARA no Miyako died (as a case of a mother's death) and Emperor Kanmu's father, Retired Emperor Konin died (as a case of a father's death).