In Roman Catholic tradition, a Jubilee is a year of penance instituted every 50 or 25 years. [1] Following the model of Augustus, the Roman emperors typically celebrated major jubilees on the 10th year of their reigns. The decennalia marked the 10th year, the vicennalia the 20th, and—in the case of Constantine the Great —the tricennalia the 30th. More @Wikipedia
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