Koshitsu Shinto is a general term for conclave rites performed by the emperor (who is now 'the symbol of the state and of the unity of the people' under the Japanese Constitution) in order to pray myriad deities centering Amaterasu Ohmikami (a goddess who is the ancestral deity of the emperor according to the Japanese myth) and the imperial ancestral deities for a long continuation of the state, for happiness of the people and for world peace, and it has an independent system. Daijosai, or Great Festival of Thanksgiving, is the first Niinamesai (a rite of Thanksgiving) performed by the newly crowned emperor of Japan in a palace called Daijokyu which is temporarily built inside the Imperial Palace. Besides this rite, rites performed at the Grand Shrine of Ise are to be included in this category, since Amaterasu Ohmikami is enshrined in there.
Niinamesai is the most important Shinto rite which is performed in order to make an offering of the first fruits of a year's grain harvest thanking the deities for their blessing and also sharing the food produced by these first grains with the deities. According to the Japanese myth, it was Amaterasu Ohmikami who performed this rite for the first time.
When the capital of Japan was transferred from Kyoto to Tokyo in 1869, three imperials hrines were erected inside the Imperial Palace. Kashikodokoro, which enshrines the imperial ancestral deity, Amaterasu Ohmikami, is centered among the three, and on the east side, there is Shinden which enshrines the deities of heaven and earth, and on the west side, Koreiden which enshrines the spirits of successive emperors. In addition to them, Shinkaden was built in order to perform Niinamesai there. These shrines are connected by corridors, and all the rites of the Koshitsu (Imperial House) Shinto are performed in these shrines.
In connection with Niinamesai, Emperor Showa (the 124th Emperor and the father of the present Emperor) started to cultivate rice in the water-field inside the Palace, doing all the procedures by himself including seeding, bedding young plants, harvesting, in order to make an offering of the harvest produced by himself to the deities.
There are clergy men and women called Shoten (men) and Nai-Shoten (women) are serving in order to assist the emperor to perform the rites. The number of rites performed by the emperor reaches several tens a year including Genshisai, the first rite of a year. There are scholars who call the emperor the king of the ritual. It is considered that the true nature of the emperor is to be always with Kami (the deities).