The calculation of the date of Easter has a fascinating history, and algorithms and computer programs abound (for example, [1], [2], [9], [10], [14], and [17]); there are also oddities such as the “finger algorithm” shown in the frontispiece of this chapter and the nomogram of Figure 9.1.Many of the computations rely on the formulas of Gauss [5], [6] (see also [8]). Our fixed-date approach allows considerable simplification of “classical” algorithms.

The history of the establishment of the date of Easter is long and complex; good discussions can be found in [3], [7], and [12]. The Council of Nicæa convened in 325 c.e. by Constantine the Great, was concerned with uniformity across various Christian groups. At the time of Nicæa, almost everyone in the official Church agreed to the definition that Easter was the first Sunday after the first full moon occurring on or after the vernal equinox [3] (a rule promulgated by Dionysius Exiguus and the Venerable Bede, who attributed it to the Council of Nicæa). By this definition, Easter is delayed one week if the full moon is on a Sunday, lessening the likelihood of its being on the same day as the Jewish Passover. This was contrary to the practice of the Quartodecimans, who celebrated Easter on the day of the full moon, 14 days into the month, regardless of the day of the week.

The concern that the date of Passover would influence the date of Easter goes back to the earliest days of Christianity. For example, Eusebius (Vita Constantini, book iii, pp. 18-20) gives a letter of the Emperor sent to those not present at the Council of Nicæa: