noun which may take the genitive case particle 'no', noun
once in an emperor's reign
A set phrase referring to an event or ceremony that occurs only once during the reign of a particular emperor. Often used in historical or formal contexts.
The Daijosai is an important ceremony that takes place only once in an emperor's reign.
Compound of 一代 (one reign) and 一度 (once). The exact historical derivation is uncertain, but the phrase is conventionally associated with imperial ceremonies.