Easter falls very late this year, on Sunday, April 23rd. Just once since the 19th century has it been later; in 1943 when it fell on the last possible date, April 25th.

One of the most frequent inquiries from the public to my office concerns the date of Easter. What most intrigues people is why it varies so much from year to year. How come, for example, that the date of Easter this year is 19 days later than in 1999?

The essential fact governing the determination of the date of Easter is that the death and resurrection of Jesus Christ occurred at the time of the Jewish feast of Passover. This is celebrated on the 14th day of the Hebrew month of Nisan, that is, at the first full moon following the spring equinox. From the beginning, therefore, the date of Easter was directly linked to the fluctuating lunar cycles.

All the Gospels record that the Resurrection occurred on a Sunday. This is why from earliest times the sacred character of the Lord's Day (Rev 1.10) was honoured by Christians, who adopted the Jewish custom of the weekly sabbath but observed it on Sunday instead of Saturday.

The first general council of the church, at Nicaea (modern Iznik, north-western Turkey) in AD325, formally sanctioned as the universal norm for Christians the already well-established custom of celebrating Easter on the Sunday following the full moon after the spring equinox.

The Roman world of that time, and also the wider world for many centuries later, followed the Julian calendar, introduced by Julius Caesar in 45BC. This was based on an average year of 365.25 days, with leap years every fourth year and in every centenary year.

It was known throughout the Middle Ages that the actual length of the year was less than was allowed for in the Julian calendar. So it became progressively out of step with the seasons.

There were numerous attempts at reform, but it was not until Pope Gregory XIII (1572-1585) set up a commission to study the issue and make recommendations that decisive action followed. He established the Vatican observatory to provide essential astronomical data.

The upshot was that when the Gregorian calendar was inaugurated in 1582, 10 days were dropped from that year as a once-off measure. The day after October 4th was designated as October 15th.

The Gregorian calendar retained the leap year every four years but dropped it in centenary years, except where they are divisible by 400. Hence, the leap year this year.

The Gregorian calendar can predict dates of full moons very accurately, more so than its designers probably knew. Not until the year 8000 will the spring equinox begin to deviate systematically by as much as half a day from its Gregorian cycle.

Also, the average length of the Gregorian calendar year is 365.2425 days, so close to the actual year that the difference between the two will not amount to a day until the year 4500. In contrast, the Julian calendar is currently out of alignment with the astronomical cycle by about 13 days.

However, Pope Gregory's experts were aware that it is impossible to devise a precise calendar. Occasionally the full moon computed in their calendar may differ from the astronomical full moon. This happened in 1962.

The earliest date of Easter in the Gregorian calendar is March 22nd and the latest April 25th, a possible spread of 35 days. However, only very infrequently does it occur on or adjacent to the outer limits.

For example, it last fell on March 22nd in 1818. The earliest Easter in the last century was on March 23rd, 1913, a date which will not recur until 2008. Only once in each of the last four centuries has it occurred on April 25th - in 1666, 1734, 1886 and 1943. In 2011 Easter will be on April 24th for the first time since 1859.

The year 2000 is, therefore, quite unusual. It is just the fifth time Easter has fallen on April 23rd; the others were 1628, 1848, 1905 and 1916. Curiously, all except 1905 were leap years.

The most frequent dates for Easter lie well between the two extremes. April 11th and 16th lead, with 18 Easters each. April 5th and 6th and March 31st each have had 17. The least frequent date is March 24th with just two (1799 and 1940).

GIVEN the central importance of the Resurrection in the history of salvation, Easter is the supreme festival of the Christian year. It is hardly surprising, then, that the date of Easter was a sensitive issue among Christians in the early centuries, sometimes provoking heated controversies, not least the dispute between the Celtic church and Rome which was famously settled in favour of Rome at the Synod of Whitby in AD664.

Early on there were a variety of methods of computing the paschal cycle, so that when Christianity spread from Palestine there could be a wide divergence in the date of Easter.

St Augustine records that in AD387 the feast was celebrated on March 21st in Gaul, on April 18th in Italy, and on April 25th in Alexandria.

However, in AD525, a Roman canonist, Dionysius Exiguus - Denis the Little - devised a table of Easter dates based on the cycle adopted by the Council of Nicaea. This table became the norm almost universally until the Gregorian reform gradually replaced it everywhere, except among the Eastern Orthodox churches.

The Orthodox churches took the view that when the Council of Nicaea formally declared how the date of Easter was to be determined it was in accordance with a cycle based on the Julian calendar and, therefore, they should continue to use that. Consequently, the Western and Orthodox dates of Easter are rarely the same.

Last year the Orthodox Easter was a week later than in the west, and this year it is on April 30th. Next year, however, both celebrations will coincide on April 15th.

Interestingly, today the Orthodox churches use the Julian calendar in relation to the date of Easter only. For all other purposes they follow the Gregorian calendar, in line with a decision of the Pan-Orthodox Council in 1923.

In more recent times discussion has tended to centre on the possibility of celebrating Easter within a more constant annual timescale. One way of doing this would be to free the date from its dependence on the moon. This would reduce the current oscillation of five weeks to one week.

An alternative would be to designate a fixed Sunday each year as Easter Day. It is not widely known today that this specific issue was discussed at the Second Vatican Council.

In an appendix to its Constitution on the Sacred Liturgy, issued on December 4th, 1963, the council recognised "the importance of the wishes expressed by many concerning the assignment of Easter to a fixed Sunday" and declared that it was not opposed to this provided the other Christian churches gave their assent.

Jim Cantwell is director of the Catholic Press and Information Office