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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.
During the festival, the people offered up vows to the emperor, called vota decennalia, for the success and perpetuity of his empire. Roman coinage was specially modified during this time to indicate the undertaking of these vows, such as with the inscription VOTA SUSCEPTA DECENNALIA, or VOTIS X. From the time of Antoninus Pius, we find these ceremonies marked on medals: PRIMI DECENNALES; SECVNDI DECENNALES; VOTA SOL. DECEN. II; VOTA SVSCEP. DECEN. III. These vows must have been made at the beginning of every tenth year, since on the medal of Pertinax, who only reigned for 4 months in 193, there are the inscriptions VOTA DECENN. and VOTIS DECENNALIBVS.
Burkhard Gotthelf Struve (1671-1738), in his Antiquitatum romanarum syntagma cap. IV, is of the opinion that these vows took the place of those the censors used to make in the times of the Republic for the prosperity and preservation thereof. In effect, they were not only made for the ruler, but also for the state, as may be observed from Cassius Dio, and Pliny the Younger.
The Decennalia was a column in the Roman Forum marking the tenth anniversary of the tetrarchy of Diocletian.