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.