The Gregorian calendar, which is the most widely used calendar in the world today, operates on a cycle of 400 years. This means that the calendar repeats itself every 400 years. As a result, the year 2025 will be the same as the year 2425, 2825, and so on.
The Gregorian calendar was introduced in 1582 by Pope Gregory XIII. It was a modification of the Julian calendar, which had been in use since 45 BC. The Julian calendar had a leap year every four years, but this caused the calendar to drift out of sync with the solar year. The Gregorian calendar introduced a new rule for leap years: a year is a leap year if it is divisible by 4, but not by 100, unless it is also divisible by 400. This rule ensures that the Gregorian calendar stays in sync with the solar year.
