Q. What is the Gregorian calendar?
A. The Julian calendar — an earlier form of our contemporary calendar — was introduced by Julius Caesar in 46 B.C. and became the standard in the West. But its imprecision allowed the true (seasonal) year to move away from the calendar year over a period of centuries. To solve this problem, in 1582 Pope Gregory XIII made adjustments to the Julian calendar to make it correspond more closely to the true length of the solar year. The new arrangement was called the Gregorian calendar.
To correct the Julian calendar, the Gregorian one features 365 days with an extra day every four years (the leap year) except in years divisible by 100 but not divisible by 400. This arrangement creates a calendar year with an average length of 365.2422 days, which is closer to the actual time it takes for the earth to make a full revolution around the sun.
When Catholic countries adopted the Gregorian calendar in 1582, a discrepancy of 10 days had accumulated between it and the Julian calendar. So the extra days were eliminated by having the date jump that year straight from October 4 to October 15. By the early twentieth century, most countries had adopted the Gregorian calendar, though the Eastern Orthodox churches continue to use the Julian calendar in their liturgy. That calendar now runs 13 days after the Gregorian one.