A fortunate coincidence

Rather fortuitously, Good Friday and the first night of Pesach (Passover) fall on the same day this year. I say fortuitously because the Last Supper that Jesus and his disciples shared is generally accepted to have been a Seder night meal, that being the meal eaten on the first night of the Pesach festival. This is not surprising bearing in mind all of Jesus's disciples, along with Jesus himself, were Jewish.

So whether you are Jewish or Christian may I wish you a hearty Chag Sameach and a Happy Easter.


The whole matter of setting the dates for Pesach and Easter is quite fascinating and a perusal of the relevant Wikipedia article does leave my head spinning slightly:

'Easter and the holidays that are related to it are moveable feasts, in that they do not fall on a fixed date in the Gregorian or Julian calendars (both of which follow the cycle of the sun and the seasons). Instead, the date for Easter is determined on a lunisolar calendar similar to the Hebrew calendar. The First Council of Nicaea (325) established the date of Easter as the first Sunday after the full moon (the Paschal Full Moon) following the northern hemisphere's vernal equinox.^[6] Ecclesiastically, the equinox is reckoned to be on March 21 (even though the equinox occurs, astronomically speaking, on March 20 in most years), and the "Full Moon" is not necessarily the astronomically correct date.
In Western Christianity, using the Gregorian calendar, Easter always falls on a Sunday between March 22 and April 25, inclusively.^[50] The following day, Easter Monday, is a legal holiday in many countries with predominantly Christian traditions.
Eastern Christianity bases its calculations on the Julian Calendar. Due to the 13 day difference between the calendars between 1900 and 2099, March 21 corresponds, during the 21st century, to April 3 in the Gregorian Calendar. Easter therefore varies between April 4 and May 8 on the Gregorian calendar (the Julian calendar is no longer used as the civil calendar of the countries where Eastern Christian traditions predominate). Among the Oriental Orthodox some churches have changed from the Julian to the Gregorian calendar and the date for Easter as for other fixed and moveable feasts is the same as in the Western church.^[51]
The precise date of Easter has at times been a matter for contention. At the First Council of Nicaea in 325 it was decided that all Christian churches would celebrate Easter on the same day, which would be computed independently of any Jewish calculations to determine the date of Passover. It is however probable (though no contemporary account of the Council's decisions has survived) that no method of determining the date was specified by the Council. Epiphanius of Salamis wrote in the mid-4th century:

...the emperor...convened a council of 318 bishops...in the city of Nicea...They passed certain ecclesiastical canons at the council besides, and at the same time decreed in regard to the Passover that there must be one unanimous concord on the celebration of God's holy and supremely excellent day. For it was variously observed by people....^[52]

In the years following the council, the computational system that was worked out by the church of Alexandria came to be normative. It took a while for the Alexandrian rules to be adopted throughout Christian Europe, however. The Church of Rome continued to use an 84-year lunisolar calendar cycle from the late 3rd century until 457. It then switched to an adaptation by Victorius of the Alexandrian rules. This table was so inaccurate that the Alexandrian rules were adopted in their entirety in the following century. From this time, therefore, all disputes between Alexandria and Rome as to the correct date for Easter cease, as both churches were using identical tables.
Early Christians in Britain and Ireland also used a late 3rd century Roman 84-year cycle. They were suspected of being Quartodecimans, unjustly because they always kept Easter on a Sunday, although that Sunday could be as early as the fourteenth day of the lunar month. This was replaced by the Alexandrian method in the course of the 7th and 8th centuries. Churches in western continental Europe used a late Roman method until the late 8th century during the reign of Charlemagne, when they finally adopted the Alexandrian method. Since 1582, when the Catholic Church adopted the Gregorian calendar while the Eastern Orthodox and most Oriental Orthodox Churches retained the Julian calendar, the date on which Easter is celebrated has again differed.

Computations

Main article: Computus

In 725, Bede succinctly wrote, "The Sunday following the full Moon which falls on or after the equinox will give the lawful Easter."^[53] However, this does not reflect the actual ecclesiastical rules precisely. One reason for this is that the full moon involved (called the Paschal full moon) is not an astronomical full moon, but the 14th day of a calendar lunar month. Another difference is that the astronomical vernal equinox is a natural astronomical phenomenon, which can fall on March 19, 20, or 21, while the ecclesiastical date is fixed by convention on March 21.^[54]
In applying the ecclesiastical rules, Christian churches use March 21 as the starting point in determining the date of Easter, from which they find the next full moon, etc. The Eastern Orthodox and Oriental Orthodox Churches
Easter is determined on the basis of lunisolar cycles. The lunar year consists of 30-day and 29-day lunar months, generally alternating, with an embolismic month added periodically to bring the lunar cycle into line with the solar cycle. In each solar year (January 1 to December 31 inclusive), the lunar month beginning with an ecclesiastical new moon falling in the 29-day period from March 8 to April 5 inclusive is designated as the paschal lunar month for that year. Easter is the third Sunday in the paschal lunar month, or, in other words, the Sunday after the paschal lunar month's 14th day. The 14th of the paschal lunar month is designated by convention as the Paschal full moon, although the 14th of the lunar month may differ from the date of the astronomical full moon by up to two days.^[55] Since the ecclesiastical new moon falls on a date from March 8 to April 5 inclusive, the paschal full moon (the 14th of that lunar month) must fall on a date from March 21 to April 18 inclusive.
Accordingly, Gregorian Easter can fall on 35 possible dates—between March 22 and April 25 inclusive.^[56] It last fell on March 22 in 1818, and will not do so again until 2285. It fell on March 23 in 2008, but will not do so again until 2160. Easter last fell on the latest possible date, April 25, in 1943 and will next fall on that date in 2038. However, it fell on April 24, just one day before this latest possible date, in 2011 and will not do so again until 2095. The cycle of Easter dates repeats after exactly 5,700,000 years, with April 19 being the most common date, happening 220,400 times or 3.9%, compared to the median for all dates of 189,525 times or 3.3%.
The Gregorian calculation of Easter was based on a method devised by the Calabrian doctor Aloysius Lilius (or Lilio) for adjusting the epacts of the moon,^[57] and has been adopted by almost all Western Christians and by Western countries who celebrate national holidays at Easter. For the British Empire and colonies, a determination of the date of Easter Sunday using Golden Numbers and Sunday letters was defined by the Calendar (New Style) Act 1750 with its Annexe. This was designed to exactly match the Gregorian calculation.

Relationship to date of Passover

In determining the date of the Gregorian and Julian Easter a lunisolar cycle is followed. In determining the date of the Jewish Passover a lunisolar calendar is also used, and because Easter always falls on a Sunday it usually falls up to a week after the first day of Passover (Nisan 15 in the Hebrew calendar). However, the differences in the rules between the Hebrew and Gregorian cycles results in Passover falling about a month after Easter in three years of the 19-year cycle. These occur in years 3, 11, and 14 of the Gregorian 19-year cycle (corresponding respectively to years 19, 8, and 11 of the Jewish 19-year cycle).
The reason for the difference is the different scheduling of embolismic months in the two cycles.

Further information: computus

In addition, without changes to either calendar, the frequency of monthly divergence between the two festivals will increase over time as a result of the differences in the implicit solar years: the implicit mean solar year of the Hebrew calendar is 365.2468 days while that of the Gregorian calendar is 365.2425 days. In years 2200–2299, for example, the start of Passover will be about a month later than Gregorian Easter in four years out of nineteen.
Since in the modern Hebrew calendar Nisan 15 can never fall on Monday, Wednesday, or Friday, the seder of Nisan 15 never falls on the night of Maundy Thursday. The second seder, observed in some Jewish communities on the second night of Passover can, however, occur on Thursday night.^{[citation needed]}
Because the Julian calendar's implicit solar year has drifted further over the centuries than those of the Gregorian or Hebrew calendars, Julian Easter is a lunation later than Gregorian Easter in five years out of nineteen, namely years 3, 8, 11, 14, and 19 of the Christian cycle. This means that it is a lunation later than Jewish Passover in two years out of nineteen, years 8 and 19 of the Christian cycle. Furthermore, because the Julian calendar's lunar age is now about four to five days behind the mean lunations, Julian Easter always follows the start of Passover. This cumulative effect of the errors in the Julian calendar's solar year and lunar age has led to the often-repeated, but false, belief that the Julian cycle includes an explicit rule requiring Easter always to follow Jewish Passover.^[58][59] The supposed "after Passover" rule is called the Zonaras proviso, after Joannes Zonaras, the Byzantine canon lawyer who may have been the first to formulate it.^[60][61]'