The Gregorian calendar is a reform of the Julian calendar. The two calendars are structured in the same way, and their months bear the same names, but they are currently thirteen days out of step with each other, and slowly moving further apart.
The mean Julian year of 365 d 6 h is slightly longer than the mean tropical year of 365 d 5 h 48 min 45 s. The Julian calendar thus falls about 11 min behind the seasonal cycle each year. The calendar gets a day behind every 128 years. When the bishops of the church met in the Council of Nicaea in AD 325, one of the tasks before them was the regulation of the date of Easter. For this they had to determine the date of the vernal equinox. Caesar had set up the Julian calendar to keep March 25 as the equinox, but 370 years later the equinox was found to be at March 21. Still today, the calculation of the date of Easter derives from setting March 21 as an imaginary constant vernal equinox.
As the centuries passed, the Julian calendar got increasingly out of synchronisation with the seasons. In the 16th century, the date of Easter was still calculated as if March 21 was the vernal equinox, but the actual equinox was occurring between March 10-12: getting earlier as the century progressed. This error was a huge embarrassment to the church, and various popes endeavoured to correct it. Pope Pius V (1566-1572) appointed Aloysius Lilius to lead a commission investigating methods of correcting the calendar. Upon his death, the prodigious Jesuit mathematician Christoph Clavius (1537-1612) took up the challenge. The main proposal was twofold. Firstly, to get the calendar back to where it should be (that is, vernal equinox on March 20/21), ten days should be dropped from the calendar. Secondly, to avoid this happening again three leap years in every four hundred years should be dropped. The commission did produce far more work than just this, including new tables for calculating New Moons and Full Moons: particularly useful for finding the date of Easter. In 1582, Pope Gregory XIII (1572-1585, after whom the calendar is named Gregorian) ordered that the ten days be dropped from October (in the papal bull Inter Gravissimas issued on 1582 February 24). Therefore, Thursday 1582 October 4 was followed by Friday 1582 October 15. It should be noted that the cycle of the days of the week was not altered. To reduce the number of leap years the Julian rule that every year divisible by four is a leap year was modified. Century years (those divisible by a hundred) became normal years of 365 d. However, all years divisible by 400 remain leap years of 366 d. These rules reduce the number of leap years from 100 to 97 out of every 400 years. Therefore the mean Gregorian year is 365,242 5 d (or 365 d 5h 49 min 12 s) long. The Gregorian year is still about 27 s longer than the mean tropical year, but that means that it will take 3220 years for the Gregorian calendar to fall a day short of the tropical year.
The Gregorian calendar was adopted first by Roman Catholic countries. Protestant countries did not feel it was appropriate to follow the lead of the Pope. However, eventually, the Gregorian calendar became normative throughout the world.
Technically, the Gregorian calendar is a Roman secular calendar rather than a religious one. However, this and the Julian calendar have played an important role in determining Christian observances through the years. Easter, and the feast which depend on it for their date, is relatively free from being tied to the calendar.
The Gregorian calendar is used throughout Western Christian groups. However, Eastern Christians use the Julian calendar or the Revised Julian calendar. The latter is a Greek revision of the former in 1924. The Revised calendar is virtually identical to the Gregorian calendar.