What does Boxing Day have to do with boxing?
late 14c., "period of fifteen years," a chronological unit of the Romans, originally for taxation purposes, fixed by Constantine and reckoned from Sept. 1, 312; it was still in use in the Middle Ages. From Latin indictionem (nominative indictio) "declaration, appointment," noun of action from past participle stem of indicere (see indictive).
in ancient Rome, the fiscal year. During the inflation of the 3rd century AD the Roman government supplied court and army employees by ordering the requisition, or by compulsory purchase (indictio), of food and clothing. Such indictiones were irregular, often oppressive, and inequitable. Reform measures under Diocletian (AD 284-305) provided for the annual levy of indictio based on land and population censuses, hence the institution of indiction, or fiscal year. From AD 287 indictions were numbered in cycles of 5 years. From 312 they were reckoned in cycles of 15 years. The indiction was reckoned from September 1, unlike the civil (consular) year, which began January 1.