The Gregorian Calendar was introduced in the Catholic parts of Europe in 1582 C.E. by Pope Gregory XIII (then the religious leader of the Roman Catholic faith) as an improvement upon the Julian Calendar to keep the average length of the calendar year better in line with the seasons.
The rules, months, and days of the Gregorian calendar are the same as those of the Julian Calendar, except for the leap year rules. In the Gregorian calendar, a year is a leap year if the year number is evenly divisible by 4, but not if the year number is evenly divisible by 100, and this last exception must not be applied if the year number is evenly divisible by 400. For example, 1600 and 2000 are leap years, but 1700, 1800, and 1900 are not.
For the other details, see Julian Calendar.