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During the Renaissance of the 16th century, the Catholic Church spearheaded a switch from the Julian to the Gregorian calendar in predominantly Catholic countries such as Spain, France, Italy, Poland and Portugal.
The Julian calendar, introduced by Julius Caesar, was imprecise, making it difficult for the Church to determine when to celebrate the Easter holiday. Citizens of these countries fell asleep Oct. 4, 1582, and woke up the next day on Oct. 15, 1582, having lost 10 days from the calendar year. But it would take nearly four centuries for the rest of the world to adopt the Gregorian calendar.
Such inconsistencies throughout the civilized world can be a headache for genealogy researchers. For example, Dec. 25, 1725, in the United Kingdom and the same date in Austria were two different days. And family historians must take the change into account when calculating birth and death dates for ancestors who lived at the time.
The map above illustrates the year each country adopted the Gregorian calendar, and the number of days dropped upon adoption—two important facts for your research.
From the October/November 2014 Family Tree Magazine