Original: The last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).
Simplified: 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.