Tenno-sai at Susanoo Jinja 31

The Tenno – Sai (Emperor Festival) is a Japanese celebration in the Shinto religion to honour past emperors.

The Emperor (tennō, literally “heavenly emperor,” formerly often called the Mikado) of Japan is the symbol of the state and of the unity of the Japanese people. He is the head of the Japanese Imperial Family. He was also the highest authority of the Shinto religion. The Imperial House of Japan is the oldest continuing hereditary monarchy in the world. In Nihonshoki, a book of Japanese history finished in the 8th century, it is said that the Empire of Japan was founded in 660 BC by Emperor Jimmu. — Wikipedia: Emperor of Japan

Ancestral worship and Japanese law