Once upon a time, Roman civilization relied on a calendar that lasted just 355 days. This meant that every so often, an extra month had to be squeezed in between February and March to keep things on track. The catch? Deciding when to add that intercalary month was entirely a political affair. The highest-ranking Roman officials — who frequently happened to be the very same pontifices in charge of calendar adjustments — had a vested interest in stretching the year longer to extend their time in power. It's easy to see, then, how periods of political chaos could lead to the repeated skipping of these extra months, throwing the entire calendar into disarray.
Nobody understood this headache better than the legendary Julius Caesar. He had been elected Pontifex Maximus, a role essentially equivalent to chief priest of the Roman religion. So when Julius Caesar made his way back to Rome in 46 BC following a period spent in Egypt, he assembled some of the most celebrated philosophers and mathematicians of the era to devise a better system. What they produced was the Julian calendar — a framework that would go on to serve the western world for over 1,600 years.
The Julian calendar, however, carried a flaw: it operated on the assumption that the average year was 365.25 days long. By the era of Pope Gregory XIII, this small error had accumulated into a noticeable drift between the calendar and the equinoxes. This mattered enormously because determining the dates for Easter depended on those very calculations. His solution, proposed in 1582, was elegantly simple — shift the average year length from 365.25 days to 365.2425 days.
Implementing this fix proved far trickier than devising it, thanks to the deep rift between Catholics and Protestants across the western world. Catholic nations were quick to embrace the Gregorian calendar, but their Protestant counterparts dragged their feet considerably. Consider this telling contrast: the Catholic Low Countries made the switch in 1582, while Utrecht, Overijssel, and other parts of the Protestant Low Countries held out until 1700.
In a delightful bit of irony, when Great Britain finally adopted the Gregorian calendar in 1752, it carefully avoided any reference to Pope Gregory XIII — as if omitting his name might somehow disguise the true origins of the change.