http://en.wikipedia.org/wiki/Gregorian_calendar http://en.wikipedia.org/wiki/Gregorian_calendar "Four Catholic countries—Spain,[18] Portugal, the Polish–Lithua

http://en.wikipedia.org/wiki/Gregorian_calendar http://en.wikipedia.org/wiki/Gregorian_calendar "Four Catholic countries—Spain,[18] Portugal, the Polish–Lithuanian Commonwealth, and most of Italy—implemented the new calendar on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October 1582."