Kalendarz gregoriański

The Gregorian calendar is the calendar that is used throughout most of the world. It began to be used from 1582. It replaced the previous Julian calendar because the Julian Calendar had an error: it added a leap year (with an extra day every four years) with no exceptions. The length of the Julian year was exactly 365.25 days (365 days and 6 hours), but the actual time it takes for the Earth to go around the Sun once is closer to 365.2425 days (about 365 days, 5 hours and 49 minutes). This difference is about eleven minutes each year.

This made the seasons get out of track, since the real first day of spring in western Europe (the equinox - day and night the same length) was happening earlier and earlier before the traditional March 21 as the centuries went by. By the 1500s, it was starting around March 11, ten days 'too early' according to the calendar. So what they did was to move the calendar forward ten days in 1582, and at the same time to make sure it did not happen again. To do this, they made an exception to the previous 'leap year rule' (add February 29 every four years). There would be no February 29 for every year that ends in 00 - unless it could be divided by 400. So the year 2000 was a leap year, because it could be divided by 400, but 2100, 2200, and 2300 will be common years, with no February 29.

It was first suggested by the Neapolitan doctor Aloysius Lilius, and was made official by Pope Gregory XIII, for whom it was named, on February 24, 1582.

Adoption

Not every country started using the new calendar straight away. Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy started to use the new calendar on Friday, 15 October 1582, following Julian Thursday, October 4 1582. The Spanish and Portuguese colonies adopted the calendar later due to the slowness of communication in those days. France adopted the new calendar on Monday, 20 December 1582, following Sunday, December 9 1582.

The Protestant Dutch provinces of Holland and Zeeland also adopted it in December of that year, but most non-Catholic countries objected to because they said the new calendar was a Catholic invention.

The Kingdom of Great Britain and the rest of the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752; by which time it was necessary to correct by eleven days (Wednesday, September 2 1752 being followed by Thursday, September 14 1752).

Source: wikipedia.org