The liturgical-calendar library provides functions to compute the dates of Easter and other Christian holidays. It uses the representation of dates from Gregor: Dates and Times. Currently, liturgical-calendar supports the calendar used primarily by churches originating in the Western half of the Roman empire: see the introduction for details.
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Reinhold Bien. Gauß and Beyond: The Making of Easter Algorithms. Archive for History of Exact Sciences 58(5), pp. 439–452, 2004. https://doi.org/10.1007/s00407-004-0078-5
Episcopal Church. The Book of Common Prayer. 1979 edition. New York: Church Publishing, 2007. https://perma.cc/BCC2-C9CY (PDF). Web version at https://bcponline.org. See in particular “The Calendar of the Church Year,” pp. 15–33 (web), and “Tables and Rules for Finding the Date of Easter Day,” pp. 880–885 (web).
Faith Wallis. Introduction, appendices, and commentary. In The Reckoning of Time, by the Venerable Bede, translated by Faith Wallis. Translated Texts for Historians volume 29. Liverpool, UK: Liverpool University Press, 1999. https://isidore.co/calibre/#book_id=7150&library_id=CalibreLibrary&panel=book_details