Simplify the following text for a Grade 13 native English-speaking person.
Original: Spain , Portugal , the Polish-Lithuanian Commonwealth , and most of Italy implemented the new calendar on Friday , 15 October 1582 , following Julian Thursday , 4 October 1582 .
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 .