The Gregorian Calendar we use today was introduced for Ecclesiastical reasons. The date of Easter had been laid down at the Council of Nicea in 325 AD. By the 16th century, Easter was occurring later in relation to the spring equinox. The idea of the Gregorian Calendar was to bring Easter back to where it would have been if the Julian Calendar in use since the Council of Nicea had been astronomically correct.

This is the calendar in use today in most countries. It differs from the Julian Calendar in its treatment of leap years and Easter.

Leap Years

Leap years occur if the year is divisible by 4 and is not a century year unless it is also divisible by 400. Therefore 1900 is not a leap year because it is a century year and is not divisible by 400, but 2000 is a leap year because it is a century year that is divisible by 400. The Julian Calendar does not include this century rule for leap years so that all century years are leap years.

History

By the sixteenth century, the date of the spring equinox had moved 10 days since the Council of Nicea in 325 AD. Then, the dating of Easter had been standardised using 21 March as the date of the vernal equinox and the Metonic cycle as the basis for calculating lunar phases.

Following the Council of Trent, in 1568 Pope Pius V introduced a new Breviary (Roman Catholic book of psalms, hymns and prayers etc recited daily by priests). Then in 1570 he introduced a Missal (Roman Catholic book containing the prayers etc of masses for a whole year). Both contained adjustments to the lunar tables and leap-year system. Pope Gregory XIII succeeded him in 1572 and decided that Pius' reforms didn't go far enough, so in 1574 he set up a commission to consider reform of the calendar.

Their recommendations were put into effect in Catholic countries on 24 February 1582 by the papal bull 'Inter Gravissimus'�. The main author of the new system was the Naples astronomer Aloysius Lilius. Ten days were deleted from the calendar so that 4 October 1582 was followed by 15 October 1582 and subsequent vernal equinoxes would occur about 21 March. A new table of New Moons and Full Moons was introduced for determining the date of Easter.

A summary of the timetable of the introduction of the Gregorian Calendar is as follows:-

1574    Calendar Commission set up
1582    Gregorian Calendar enacted in Catholic countries
1700    Gregorian Calendar introduced in Protestant parts of Germany
1752    Gregorian Calendar introduced in Great Britain & its Colonies (including the United States)
1873    Gregorian Calendar introduced in Japan
1918    Gregorian Calendar introduced in Russia
1927    Gregorian Calendar introduced in Turkey
1949    Gregorian Calendar introduced in China

Christmas Day

Christmas Day is celebrated on 25 December.

Until 1923 Eastern Orthodox Churches celebrated Christmas on 25 December in the Julian Calendar, but then the Ecumenical Patriarch Meletios IV convened a congress at which it was decided to use the Gregorian date instead for this one festival.

Easter Day

Easter Day was fixed in 325 AD by the Council of Nicea to occur on the first Sunday after the first full moon (14/15 days after the new moon) occurring on or after the vernal equinox. This causes it to be delayed for one week if the full moon is on Sunday to reduce the chances of it occurring on the same day as the Jewish Passover. The full moon is not the true astronomical full moon but "the Paschal moon" or "the ecclesiastical Full Moon" based on tables that do not take into account the full complexity of lunar motion.

The date of Easter as enacted by Dionysius Exiguus in 525 AD is based on the assumption that the vernal equinox is always on 21 March, and on lunar epacts which are ecclesiastical approximations of the lunar phases based on the 19-year Metonic cycle that is named after the Greek astronomer Meton who in 432 BCE published his observation that 19 solar years contain almost exactly 235 lunations.

The calculation of Easter in the Gregorian Calendar differs from that in the Julian Calendar by incorporating a more accurate approximation to the lunar phases developed by the German Jesuit astronomer Christoph Clavius from the suggestions made by Lilius. There are two corrections and two adjustments not made in the Julian Calendar.

  • One day is subtracted for each non-leap century year to compensate for the shift of one day forward in the date of the full moon caused by the Gregorian leap-year rule.
  • One day in 8 of every 25 century years is added to the epact to correct for the excess of every 19 Julian years of 365.25 days over 235 mean lunations. One day was also added to the initial value of 5 in the sixteenth century to minimise coincidences of Easter and Passover.
  • If the ecclesiastical full moon occurs on 19 April it is adjusted by one day to 18 April to keep it within 29 days from the vernal equinox of 21 March.
  • If the ecclesiastical full moon is adjusted to 18 April according to rule 3 then a further adjustment of one day may be necessary in the second half of a 19-year cycle to prevent the date of the Easter moon repeating within a single 19-year cycle.

Other dates

Start of Lent

Start of Lent is 46 days before Easter Day.

Passion Sunday

Passion Sunday is the Sunday 14 days before Easter Day.

Palm Sunday

Palm Sunday is the Sunday before Easter Day.

Good Friday

Good Friday is the Friday before Easter Day.

Ascension Day

Ascension Day is 39 days after Easter Day.

Pentecost (Whitsunday)

Pentecost (Whitsunday) is 49 days after Easter Day.

First Sunday in Advent

The First Sunday in Advent is the Sunday closest to November 30.