== The Nicene Formula ==
The computational system that was eventually worked out derives from the calendrical experiments made at Alexandria beginning in the mid-3rd century. According to this system, Pascha is first Sunday following the date of the Paschal Full Moon ("PFM") for a given year. The PFM is not, however, as commonly thought, the first full moon following the vernal equinox. Rather, the PFM is the first Ecclesiastical Full Moon ("EFM") date that falls on or after March 21 (or, what is the same thing, the first Ecclesiastical Full Moon that follows March 20). Ecclesiastical Full Moons are calendar dates that approximate astronomical full moons using a cycle that repeats every 19 years. March 21 is the date used for determining the PFM because it was
the near the date of the vernal equinox in the late 3rd and early 4th century A.D., when the Paschal cycle was first being developed. This formula is called Nicene because some commentators in later generations attributed it to the Nicene council.
=== The Zonaras Proviso ===
The decision of the Nicene council concerning Easter was that it should be computed independently of any Jewish computations: Christians were to compute their own, Christian, Nisan, and set Easter to its third Sunday, rather than setting it to the third Sunday in Jewish Nisan. Hence a paschalion that is consistent with Nicene principles cannot have any built-in dependence on the Hebrew calendar. Nevertheless, at least since the 12th century
is has been widely believed that Christian Easter is required always to follow, never coincide with, the first day of Unleavened Bread (Nisan 15) in the Hebrew calendar. By the 11th century the errors in the Julian calendar's equinox date and age of the moon had accumulated to where Easter did, in fact, always follow Jewish Nisan 15. This state of affairs continues to the present day, even though the Hebrew calendar suffers from a slight solar drift of its own, since the Julian calendar's errors accumulate more rapidly than the Hebrew calendar's. The 11th-12th century canonist [[Joannes Zonaras]] seems to have been the first to state the principle that Easter must always follow Hebrew Nisan 15. The principle is called the Zonaras proviso after him.
== Implementation ==
Pascha is always the Sunday ''following'' the Paschal Full Moon. Since the PFM is simply the 14th day of the Paschal lunar month, this means that Pascha is the third Sunday in the Paschal lunar month, and can fall on any date in the lunar month from the 15th (the day after the PFM) to the 21st (seven days after the PFM). That the structure of the Paschal lunar month is
modeled on that of the scriptural month of 'Aviv (now called Nisan) should be clear. The Paschal lunar month is analogous to the month of 'Aviv. It is in effect a Christian 'Aviv or Nisan. The 14th day, the Paschal Full Moon, is analogous to the day of the Passover sacrifice, and the third week, the 15th to the 21st, the week whose Lord's Day is Pascha, is analogous to the Week of Unleavened Bread.
==Shortcomings of the Julian Paschalion==