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Accession is the attainment or acquisition of a position of rank or power, typically that of monarch or president. Eg."the queen's accession to the throne".

Accession day is a celebration. It is usually the anniversary of when a monarch takes office. The earliest record of accession celebrations are from the reign of Queen Elizabeth I of England.

Accession is not the same as coronation or inauguration. Coronation and inauguration are both ceremonies which take place after the accession.