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Count the days of October 1582 in Rome . They go one , two , three , four , and then fifteen . Thursday 4 October was followed by Friday 15 October . Julius Caesar introduced the old calendar in 45 BC . Every fourth year had an extra day , so the calendar year averaged 365.25 days . The cycle of the seasons takes about 365.2422 days , so the calendar year was about 11 minutes too long . Eleven minutes sounds harmless , but it adds up to a full day in roughly 128 years . In 325 , the year of the Church council at Nicaea , the spring equinox fell around 21 March . By 1582 , the equinox was arriving around 11 March . That mattered to the Church , because Easter is counted from the spring equinox . In February 1582 , Pope Gregory XIII ordered a reform of the calendar . The plan came from Luigi Lilio , a doctor and astronomer from Calabria , and the Jesuit mathematician Christopher Clavius helped to complete it . First , ten days were dropped , which brought the equinox back to around 21 March . Second , the leap - year rule changed . Every fourth year is still a leap year , but a year ending in 00 now qualifies only if it can be divided by 400 . So 1600 and 2000 were leap years , but 1700 , 1800 and 1900 were not . Not every country changed in 1582 : the Papal States , Spain and Portugal did in October , and France followed in December . Britain and its colonies changed in 1752 , when 2 September was followed by 14 September . By then the gap had grown to eleven days , because 1700 was a leap year only in the old calendar . Russia changed in 1918 , when 31 January was followed by 14 February . Count October 1582 again : four , then fifteen . Those ten missing days brought the calendar back to the seasons .
