CAPUT XXII. Festorum mobilium inventio. (Chapter 22. Dates for Moveable Feastdays)

The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.' 'Because the date of Easter is a function of the date of the (northern hemisphere) spring equinox, the Catholic Church considered unacceptable the increasing divergence between the canonical date of the equinox and observed reality. Easter is celebrated on the Sunday after the ecclesiastical full moon on or after 21 March, which was adopted as an approximation to the March equinox....The reform adopted was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio).' 'Lilius's work was expanded upon by Christopher Clavius in a closely argued, 800-page volume [*Novi calendarii romani apologia*. Rome, 1588]. He would later defend his and Lilius's work against detractors. Clavius's opinion was that the correction should take place in one move, and it was this advice that prevailed with Gregory.' -- Wikipedia, '[Gregorian Calendar](https://en.wikipedia.org/wiki/Gregorian_calendar)' (accessed January, 2022). ---------------------- Included here are excerpted pages from the table found in '*Caput XXII. Festorum Mobilium Inventio*,' pp. 375-504, from the work *Romani Calendarii a Gregorii XIII. P.M. Restituti*. *Romani Calendarii* is itself part of Vol. 5 of *[Opera mathematica of Christoph Clavius](https://curate.nd.edu/show/3f462516g78)*, *Moguntiae: excudebat Joannes Volmari*, 1612. The table presents the dates calculated for Easter, and other moveable feastdays, from A.D. 1600 to 5000. The year 2020 is shown as an example, with the date being calculated as April 17. ---------------------- p. 375 (bottom). CAPUT XXII. Festorum mobilium inventio. Praecipua festa mobilia in Ecclesia Catholica sunt Septuagesima, Quadragesima sive dies Cinerum, Pascha, Ascensio Domini, Pentecoste, Corpus Christi, Dominica prima Adventus: quibus in tabulis festorum mobilium adjungiter etiam numerus Dominicarum inter Pentecosten, et Adventum Domini, tanquam necessarius iis, qui Divinum officium recitant, ut nimirum sciant, quotnam Dominicae ex iis, quae post Epiphanium ponuntu, interferendae sint inter Dominicam 23. et 24. post Pentecosten. Quia vero in Calendario Gregoriano perpetuo, quod sub initium hujus operis proposuimus, copiose satis Canon 6. inventione harum festivitatem mobilium egimus, propositis duabus tabulis generalibus ad eam rem perutilibus, una antiqua, et nova altera, non repetemus hoc loca ea, quae ibi scripsimus i sed priorem duntaxat resolutam in annos communes, ex bissextiles alio modo, duplicatam nimirum, exhibemus, ut sine molestia ulla, sive in annis communibus, sive bissextilibus, mobilia festa invenire possint: qua quidem tabula resoluta Ecclesia Orientalis uti consuevit…. p. 379 (bottom). Sequitur tabula Festorum Mobilium ab anno 1600. Usque ad annum 5000. p. 394 (middle). Anno Domini – 2020: Pasch Calend. Novi – 12 Aprilis