Once upon a time, the Romans used a 355-day calendar. As a result, they had to insert an intercalary month between February and March from time to time. The problem was that this insertion was a political process. After all, a longer year meant a longer term for the highest Roman officials, who were often one-and-the-same as the pontifices responsible for such matters. As such, the repeated omission of intercalary months could make for a horrible mess in times of political turmoil.
The famous Julius Caesar was very familiar with this problem. He had been elected the Pontifex Maximus, who could be considered the chief priest of the Roman religion. Due to this, when Julius Caesar returned to Rome in 46 BC after having spent some time in Egypt, he called in some of the best-known philosophers and mathematicians of the time to create a new calendar. This was the Julian calendar, which went on to be used by the western world for more than 1,600 years.
Unfortunately, the Julian calendar had an issue in that it assumed the average length of the year to be 365.25 days. By the time of Pope Gregory XIII, this had produced a significant drift of the calendar from the equinoxes, which was a huge problem because of the calculations involved in coming up with the dates for Easter. To fix this, he proposed a change from an average length of 365.25 days to 365.2425 days in 1582.
The problem was that the western world was divided between the Catholics and the Protestants at the time. As such, while Catholic countries rushed to adopt the Gregorian calendar, Protestant countries took much longer to do this. To name an example, the Catholic Low Countries adopted the Gregorian calendar in 1582. In contrast, Utrecht, Overijssel, and other parts of the Protestant Low Countries didn’t do so until 1700.
Amusingly, even when Great Britain adopted the Gregorian calendar in 1752, it made no mention of Pope Gregory XIII as though that would conceal what was happening.