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 .