Decennalia

Decennalia (or decennia, from Latin decennium - "tenth anniversary"; decem, "ten" + annus, "year") were Ancient Roman festivals celebrated with games every ten years by the Roman emperors.

This festival owed its origin to the fact that in 27 BC, Augustus refused the supreme power offered to him for life. Instead, he would only consent to accepting power for ten years; during the festival, he would surrender up all of his authority to the hands of the people, who, filled with joy, and charmed with the goodness of Augustus, immediately delivered it back to him again. The memory was preserved to the last ages of the empire by Decennalia, which was solemnised by subsequent emperors every tenth year of their reign, although they had received the imperium for life, and not for the limited period of ten years.