It's not Easter til it's Easter

Few things could be as confused, or as disputed for two centuries, as the date of Easter.

The easy part: Easter celebrates the resurrection of Jesus, which his followers believe occurred on the third day after his death by crucifixion some time in the period AD 27 to 33.

The slightly less easy part: Knowing that date of the Resurrection depends upon knowing the date of the Last Supper. From Mathew, Mark, and Luke, the Last Supper is clearly on Thursday (now Holy Thursday), with the Last Supper being the Passover Seder or ritual feast which takes place on the first evening of the Jewish holiday of Passover (15th day of Nisan on the Hebrew calendar). The Gospel of John can be interpreted to place the Last Supper on 14 Nisan, which would be either a Wednesday evening or a Thursday afternoon (Jewish days begin at dusk 'the day before'). Local customs varied but Christians in the west tended to use 15 Nisan for the date of the Last Supper while Christians in the east used 14 Nisan.

The much less easy part: The early Christians, who were mostly Jewish by birth, tended to celebrate a Christian Passover rather than an "Easter" or Resurrection Day. This Christian Passover was on 14 Nisan, the date of the killing of the Passover or sacrificial lamb (the symbolism of Jesus as Lamb of God was intended). As gentiles took control of the growing faith, and sought to distinguish it from it's Jewish origins, and to incorporate their virulent anti-Jewish prejudices into Christian dogma, the Christian Passover was severed from the Jewish feast. Not surprisingly, what we call "Easter" in English is referred to by a name derived from the Hebrew for Passover (Pascha) in many languages: (partial list)

• Albanian Pashkët
• Danish Påske
• Dutch Pasen or paasfeest
• Esperanto Pasko
• Finnish Pääsiäinen
• French Pâques
• Icelandic Páskar
• Indonesian Paskah
• Irish Cáisc
• Italian Pasqua
• Lower Rhine German Paisken
• Norwegian Påske
• Polish Pascha
• Portuguese Páscoa
• Romanian Paşte
• Russian Пасха (Paskha)
• Scottish Gaelic Casca
• Spanish Pascua
• Swedish Påsk
• Welsh Pasg

The sort of hard part: Since 14 and 15 Nisan would be on different days of the week each year, so would Easter. The Bishop of Rome thought Easter should always be on a Sunday, and excommunicated anyone who disagreed. There was no end to the arguments so the Roman Empirer Constantine convoked the First Council of Nicaea, in 325 A.D. The Council said, in effect, it's Sunday and only Sunday. This was not a surprise since Constantine was a Mithrasim, for whom Sunday was the holy day of the week. [The Council also prohibed self-castration -- scary that "just say 'No'"should have even been necessary.]

The really hard part: The Council of Nicaea said Easter should be determined using a set of special tables based on the Julian Calendar. The tables were revised many times until they were replaced by entirely new tables when the Gregorian Calendar was adopted in 1582. By the 1700's, most of western Europe had adopted the Gregorian Calendar. The Eastern Christian churches, however, still determine the Easter dates using the older Julian Calendar method. The usual statement, that Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox, is not a precise statement of the actual ecclesiastical rules. The full moon involved is not the astronomical Full Moon but an ecclesiastical moon (determined from tables) that keeps, more or less, in step with the astronomical Moon.

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.

resulting in that Easter can never occur before March 22 or later than April 25. The Gregorian dates for the ecclesiastical full moon come from the Gregorian tables. Therefore, the civil date of Easter depends upon which tables - Gregorian or pre-Gregorian - are used. The western (Roman Catholic and Protestant) Christian churches use the Gregorian tables; many eastern (Orthodox) Christian churches use the older tables based on the Julian Calendar. [The formula for calculating Easter using the Gregorian calendar and the ecclesiastical full moons is given below.]

The part you might not like: In the English-speaking world, Resurrection Day is called "Easter" --"Ostern" in German. The usual etymology for "Easter" is that the name is derived from Eostre or Eoster, an Anglo-Saxon goddess of spring, to whom the month of Eosturmonath, corresponding to our April, was dedicated. The explanation is that Eosturmonath began to be regarded as the Pascal Month and, therefore, that the principal feast during Eosturmonath would give it's name to the principal feast in the Pascal Month. Eoster was the goddest of the dawn and her name is also the origin of the word East.

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From the U.S. Naval Observatory:

Computing the Date of Easter

The rule is that Easter is the first Sunday after the first ecclesiastical full moon that occurs on or after March 21. The lunar cycles used by the ecclesiastical system are simple to program. The following algorithm will compute the date of Easter in the Gregorian Calendar system.

The algorithm uses the year, y, to give the month, m, and day, d, of Easter. The symbol * means multiply.

Please note the following: This is an integer calculation. All variables are integers and all remainders from division are dropped. For example, 7 divided by 3 is equal to 2 in integer arithmetic.

    c = y / 100
  n = y - 19 * ( y / 19 )
  k = ( c - 17 ) / 25
  i = c - c / 4 - ( c - k ) / 3 + 19 * n + 15
  i = i - 30 * ( i / 30 )
  i = i - ( i / 28 ) * ( 1 - ( i / 28 ) * ( 29 / ( i + 1 ) )
      * ( ( 21 - n ) / 11 ) )
  j = y + y / 4 + i + 2 - c + c / 4
  j = j - 7 * ( j / 7 )
  l = i - j
  m = 3 + ( l + 40 ) / 44
  d = l + 28 - 31 * ( m / 4 )

For example, using the year 2010,
y=2010,
c=2010/100=20,
n=2010 - 19 x (2010/19) = 2010 - 19 x (105) = 15, [see note above regarding integer calculations]
etc. resulting in Easter on April 4, 2010.