The Julian calendar reform

The earlier Roman calendar relied on additional months to stay near the seasons, but intercalation was applied irregularly. Priests administered the process, and political and administrative disruption contributed to the drift. In 46 BCE Caesar organized a reform and lengthened the transitional year to restore the position of the months. New rules took effect in 45 BCE. Festivals, official terms and dated agreements then had to operate within the revised calendar.

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The new solar calendar assigned 365 days to an ordinary year and one extra day in every four years, giving an average of 365.25 days. Fixed month lengths allowed dates to be known in advance. After Caesar’s death the leap rule was initially misapplied and later corrected under Augustus. Roman documents continued to count backward from named points within a month, so a new year structure did not immediately replace older ways of expressing individual dates.

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The Julian calendar endured through Roman government and later churches and kingdoms. Local conventions still differed over year beginnings, eras and festival names. Its average year is slightly longer than the seasonal year, causing the equinox to shift gradually toward earlier calendar dates. The Gregorian reform addressed this accumulated difference in 1582. Some religious traditions retain Julian reckoning, so the two systems continue to appear together in modern observance.

References: [1]