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Aprilis or mensis Aprilis (April) was the fourth month of the ancient Roman calendar, following "Martius" (March) and preceding "Maius" (May). On the oldest Roman calendar that had begun with March, "Aprilis" was the second of ten months in the year. April had 29 days on calendars of the Roman Republic, with a day added to the month during the reform in the mid-40s BC that produced the Julian calendar.