Have you ever had a really bad week? One that you just wanted to go to sleep and wake up when it was all over?
On this day in history in 1582, ...well, there was no October 10th, 1582 in certain parts of Christendom. That year, people went to bed on the 4th, and when they woke up, it was the 15th. It wasn't because of any sinister magic or time travel. It was because of the church.
The Council of Nicaea decided in 325 that Easter should be celebrated by all Christians on the same day. But the Julian calendar was messed up, and people in different areas clung to their traditions, and to make a long story short, in 1563 Pope Gregory XIII was like, "Guys, this is ridiculous. We've gotta get this Easter thing done. It's been over a thousand years. Seriously."
So after several years of doing some math and moving things around and arguing, scholars discovered that what they thought tomorrow's date was was actually 10 days later. They asked the pope what they should do now, and he scoffed, "Why are you guys even asking me this? If tomorrow's supposed to be the 15th, then it'll be the 15th. Come on, that's, like, not even a hard question." They were so in awe of his pontiff powers that they named the new method of counting time after him: the Gregorian calendar.
Okay, so that’s glossing over years of bureaucracy and giving the pope a much more flippant air than he probably had, but it gets the point across. The cool part about the implementation of the Gregorian calendar is that the pope didn't actually have the authority to do it: it was a civil government thing, not a church thing. But naturally when the order was sent out, the Catholic parts of the world immediately implemented it. Other countries eventually did, resisting because of one reason or another.England didn't initially want to because they didn't want to be seen as returning to Catholicism. In Slovak lands where the people were resisting Hapsburg authority didn't want to because, well, they were resisting, and why accept some things that the monarchy tries to inflict on you and not others? Long after Pope Gregory XIII died, countries continued the slow march of progress in implementing the use of the Gregorian calendar. Greece finally started using it in 1923 after four hundred years of nonverbally reminding the Roman Catholic church that it couldn't tell the Greek Orthodox Church what to do. I think we can safely assume that Pope Gregory was hanging out the whole time going, “You guyyyyys. Come onnnnn.”