The reason the Christian celebration shifts on the calendar is that it is determined in respect to the Vernal Equinox (date when night and day are of equal duration). Because of this linkage to the cycles of the Sun and Moon, Easter has always been a rite of Spring. The word Lent is from the Anglo-Saxon word lengten, which means spring.

The ecclesiastical rules are:

• Easter falls on the first Sunday following the first ecclesiastical full moon that occurs on or after the day of the vernal equinox;
• this particular ecclesiastical full moon is the 14th day of a tabular lunation (new moon); and
• the vernal equinox is fixed as March 21.
• If the full moon is on a Sunday, Easter is the Sunday after. This is because it would otherwise coincide with the Jewish Passover. Celebrates Christ's resurrection.

resulting in that Easter can never occur before March 22 or later than April 25.

The date of Easter is primarily used for liturgical purposes. Up to the eighth century there was no uniform method for determining the date of Easter but the method favoured by the Council of Nicaea (present-day Turkey). in (325 AD) gradually became the accepted method.  The current Gregorian ecclesiastical rules that determine the date of Easter trace back to the First Council of Nicaea convened by the Roman Emperor Constantine. At that time the Roman world used the Julian Calendar (put in place by Julius Caesar (46 B.C.) . Prior to the Council of Nicea , Christians in general would celebrate Easter on the 14 Nissan, whichever day that was, which is when Jews celebrate the Passover but it was felt that such an important holiday should be separated from this high holy Jewish day.

Although the Julian or solar-based calendar of the Roman Empire had been in place since 45 B.C., it did not supplant the lunar calendar that was the chart and compass of 2,000 years of Jewish history. (A lunar year is 12 lunar cycles of 29.53 days each or 354.36 days while a Julian year is 365.25 days with a leap day every four years.) The Julian calendar functions by having three years of 365 days and one year of 366 days every four years.

For the year: , the date of Easter is:

This calculator computes the date of Easter for any year after 1582 AD. The JavaScript program was adapted from a BASIC program which appeared in the Astronomical Computing column of Sky & Telescope, March, 1986. Carnaval.com got it here

Christ in Glory
Detail from Disputa dei Sacramento - The Triumph of the Christian Faith
Raphael (ca 1508) Stanza della Segnatura, Apostolic Palace, Vatican

The "popular" rule (for Roman Catholics and most Protestant denominations) is that Easter is on the first Sunday after the first full moon after the March equinox. The actual rule is similar, except that the astronomical equinox is not used; the date is fixed at March 21. And the astronomical full moon is not used; an "ecclesiastical" new moon is determined by adopted tables based on the Metonic cycle, and "full" is taken as the 14th day of that lunation. There are auxiliary rules that make March 22 the earliest possible date for Easter and April 25 the latest. The reasons for this are that the method is then independent of longitude on the Earth and is thus independent of Time Zone. It also allows the date of Easter to be calculated in advance regardless of the actual motion of the Earth around the Sun.

In the mid-1200's, an English Friar named Roger Bacon observed that the date of Easter, in addition to its prescribed wandering, was drifting farther and farther into the spring season. Astronomers now knew that the length of the solar year was closer to 365.242 days than to the 365.250 days assumed in the Julian calendar year. In 1,000 years, the Julian calendar counted 365,250 days, while in actuality, 365,242 solar days had elapsed. Bacon realized that each Julian year "overflowed" slightly into the next solar year, and that any given date was farther along in real time than the calendar would imply. Though the small mismatch caused a shift of only 11 minutes per year, this had accumulated—from Julius Ceasar's to Roger Bacon's day—into a troubling 9 days.

By the mid-1500s, Pope Gregory XIII recognized the consequences of the drift and entrusted a solution to a Jesuit mathematician and astronomer, Christopher Clavius. The enlightened Pope endorsed Clavius's findings in 1563 at the Council of Trent, and 19 years later, on October 4, 1582, Gregory signed a papal bull promulgating the new calendar that bears his name—the Gregorian calendar.

 One major difference between the Julian and Gregorian Calendar is the "leap year rule". Although it proceeded at a slow pace, by the 1700's, most of western Europe had adopted the Gregorian Calendar. The Eastern Christian churches, such as the Greek Orthodox,  still determine the Easter dates using the older Julian Calendar method.

In a congress held in 1923, the eastern churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical Full Moon for the meridian of Jerusalem. However, a variety of practices remain among the eastern churches. One exception to this is in Finland, where the Orthodox Churches have the permission to observe Easter the same time as Roman Catholics and Protestants do. Others retain the Julian calendar which in the year 2100 A.D. will be 14 days behind the Gregorian calendar

There are three major differences between the ecclesiastical system and the astronomical system.

• The times of the ecclesiastical full moons are not necessarily identical to the times of astronomical Full Moons. The ecclesiastical tables did not account for the full complexity of the lunar motion.
• The vernal equinox has a precise astronomical definition determined by the actual motion of the Sun. It is the precise time at which the apparent longitude of the Sun is zero degrees. This precise time shifts within the civil calendar very slightly from year to year. In the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.
• The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The vernal equinox occurs at a specific date and time all over the Earth at once.

The calendar year is 365 days long, unless the year is exactly divisible by 4, in which case an extra day is added to February to make the year 366 days long. If the year is the last year of a century, for example 1700, 1800, 1900, 2000, then it is only a leap year if it is exactly divisible by 400. Therefore, 1900 wasn't a leap year but 2004 was. The reason for these rules is to bring the average length of the calendar year into line with the length of the Earth's orbit around theSun, so that the seasons always occur during the same months each year.

The two calendars do coincide fairly often. Both will have Easter fall on the same day in 2007, 2010, 2011, 2014 and 2017.

Ascension Day, the fortieth day of Easter (39 days after Easter Sunday), is a Thursday. Pentecost, the fiftieth day of Easter, also called Whitsunday, is always 10 days after Ascension Day, hence the seventh Sunday after Easter Sunday. It ends the Easter season and begins the season of Pentecost

The season of Advent begins on Advent Sunday, which is the fourth Sunday before Christmas. It can occur anywhere from 27 November to 3 December. Christmas Day is 25 December, but Christmas is 12 days long (think of the song), followed by Epiphany, 6 January. The length of Epiphany varies because it is dependent upon the date of the upcoming Easter.

People throng to visit 'seven churches' on Maundy Thursday. Christians kneel, reflect and pray beside the tomb of Christ. Good Friday gives a sombre outlook where churches are deprived from the traditional ornamental style for a single day. Red resembling the Blood of Christ is splashed all over the place. The situation changes completely the next day in the evening. Celebrations start in pitch darkness. They are then illuminated by flickering candle lights. Finally there is an 'explosion' of light where churches are suddenly illuminated with candles, chandeliers, bulbs, floodlights etc. Bells toll happily as they break the night's normal silence in order to announce Christ's central event - resurrection, exactly when the singing of the 'Glorja' commences.

maltamedia.com/easter

maltamedia.com/easter-mejda MaltaMedia's seasonal feature for Holy Week and Easter in Malta highlights traditional, religious, astronomical, and gastronomical aspects of the greatest celebration in the Christian calendar. Whether you're after an understanding of why the date for Easter Sunday changes each year or the recipe for Figolli you'll find it all in this feature.