In 1582, the Gregorian Calendar was adopted for the first time with Poland, Portugal and Spain leading the way. The calendar was implemented by and named after Pope Gregory XIII . He didn’t like the Julian Calendar in use at the time because Easter kept creeping back to an earlier time of the year so that eventually it would fall in the middle of winter, even on Christmas Day (which wasn’t creeping) and really confuse Christians.
Jews didn’t care a whole lot because they were already up to year 5303. Muslims were back at 1001. The Chinese were celebrating 4278 (Year of the Horse). Certain scientists were way ahead of everyone else at 11582, having added 10,000 years to the current year to make the starting point the beginning of the human era (like they knew what day that was) instead of the birth of Jesus who they say wasn’t born in 1 BC but in 4 BC (lied about his age) and wasn’t born on his birthday (Christmas).
But back to the Christians for whom it was October 4, 1582, and also for whom tomorrow would be October 15, because Pope Gregory took ten days right out of the calendar to put Easter back where it had been when he was a boy. Well, you can just imagine how upset folks who had birthdays or special anniversaries or doctors’ appointments between October 5 and 14 were.
Some people just refused to use the new calendar. Many European countries fell into line later in the year. But many Protestant countries thought the new calendar was part of a Catholic plot . Britain (and its colonies) didn’t come along until 1752. The Greeks didn’t start using it until 1923. And a few malcontent members of the U.S. House of Representatives still haven’t adopted it (or a budget).