The Days of the Moon

by Fr. William Rock, FSSP

There is a plethora of Catholic calendars with varying degrees of detail and information.  Interestingly, some of them, such as those published by the Sons of the Most Holy Redeemer and Sophia Press, indicate the phases of the moon.  The question naturally arises as to why such calendars include this information.  Some might even object that such a thing smacks of paganism.  The answer, however, has its roots in the calendar of the Old Testament.

The Hebrew people used a lunisolar calendar where the length of the 12 months was determined by the cycle of the moon, resulting in months which were 29 or 30 days long (the lunar part of the calendar).  As this would fall short of the solar year, an additional month would be added in intervals to realign the lunar months with the solar year (the solar part of the calendar).  The month would start with the new moon,¹ which is when the moon is at its minimum visibility at the start of a period of increase (i.e., the first visible crescent when the moon begins to wax).  The days of the month, for their part, were counted, inclusively, from the most recent new moon.  The days of the month, then, are really days of the moon, with the first day of the moon, the first day of the month, corresponding to the new moon.

The beginning of the month, the new moon, was kept as a festal day.  God commanded the Hebrew people through Moses that “on the first days of your months [that is, the day of the new moon], you shall sound the trumpets over the holocausts, and the sacrifices of peace offerings, that they may be to you for a remembrance of your God.  I am the Lord your God” (Num 10:10).  Again, in the Psalms is found: “blow up [bucinate, sound] the trumpet on the new moon, on the noted day of your solemnity” (8o:4).

The feasts of the Torah, which were to be kept on certain days of the month, were determined by counting days from the start of their respective months, that is from the new moon which began the month.  The most important of these feasts for our purposes here is the Feast of Passover which was to be kept in “the first month, the fourteenth day of the month at evening” (Lev 23:5).  It is important to note here that the full moon occurs “about 14 days”² after the new moon.  This means that the celebration of the Passover was associated with, but not determined by, the full moon.

In the early Church, there was a controversy over when the feast of the Lord’s Resurrection, the Feast of Easter, should be kept.  The greater number of the churches kept this feast on Sunday, while the churches of Asia Minor kept the feast on the fourteenth day of the first Hebrew month regardless of which day of the week it fell.  This controversy came to an end at the First Ecumenical Council of Nicaea.  The Old Catholic Encyclopedia explains as follows:

It is generally held that the Last Supper took place on the Jewish Feast of the Passover, which was always kept on the fourteenth day of the first month of the old Jewish calendar.  Consequently, since this month always began with that new moon of which the fourteenth day occurred on or next after the vernal equinox, Christ arose from the dead on Sunday, the seventeenth day of the so-called paschal moon.  It is evident, then, that an exact anniversary of Easter is impossible except in years in which the seventeenth day of the paschal moon falls on Sunday.  In the early days of Christianity there existed a difference of opinion between the Eastern and Western Churches as to the day on which Easter ought to be kept, the former keeping it on the fourteenth day and the latter on the Sunday following.  To secure uniformity of practice, the Council of Nicæa (325) decreed that the Western method of keeping Easter on the Sunday after the fourteenth day of the moon should be adopted throughout the Church, believing no doubt that this mode fitted in better with the historical facts and wishing to give a lasting proof that the Jewish Passover was not, as the Quartodeciman heretics believed, an ordinance of Christianity.³

The calendar being used by these Christians was the Julian Calendar, the months of which, by this time, no longer had any ties to the lunar cycles.  Resulting from the need to determine the relevant “fourteenth day of the moon,” the early Christians adopted the Greek Metonic Lunar Cycle, also known as the Cycle of Golden Numbers, founded on the work of Meton of Athens (5th century B.C.).  A key part of Meton’s work is summarized as follows:

In the year now known as 432 B.C., Meton, an Athenian astronomer, discovered that 235 lunations (i.e. lunar months) correspond with 19 solar years, or, as we might express it, that after a period of 19 solar years the new moons occur again on the same days of the solar year.  He therefore divided the calendar into periods of 19 years, which he numbered 1, 2, 3, etc. to 19, and assumed that the new moons would always fall on the same days in the years indicated by the same number.  This discovery found such favour among the Athenians that the number assigned to the current year in the Metonic Cycle was henceforth written in golden characters on a pillar in the temple, and, whether owing to this circumstance or to the importance of the discovery itself, was known as the Golden Number of the year.⁴

And so, the Nicaean calculation of Easter was based not only on the Julian determination of the Spring Equinox (solar), which was observed on March 21st, but also the relevant 14th day of the moon of the Metonic Lunar Cycle (lunar), which, as was stated above, is associated with the full moon.  The Church, then, not only had a solar calendar in the Julian, but also an independent lunar calendar.

The method for determining Easter established by the Council of Nicaea is generally summarized as follows: “Easter falls on the first Sunday following the first full moon following the Spring Equinox.”  And, while this is helpful, it should be more actually expressed as: “Easter is to be celebrated on the first Sunday following the 14th day of the Moon of the Metonic Lunar Cycle which occurs after March 21st of the Julian Calendar, when the Spring Equinox is observed.”  Because there might be some discrepancy between the astronomical events and their observances in the calendars, in order to avoid confusion, the Julian March 21st is referred to as the Paschal or Ecclesiastical Equinox, while the associated 14th day of the moon is referred to as the Paschal or Ecclesiastical full moon.  As such, a more proper rendering of the first would be: “Easter falls on the first Sunday following the first Ecclesiastical full moon following the Ecclesiastical Spring Equinox.”

At the time of the First Council of Nicaea, the astronomical Spring Equinox did regularly occur on or around the Julian date of its observance and the astronomical Paschal full moon regularly occurred on or around the associated 14th day of the moon.  However, as the centuries progressed, the shortcomings in the Julian Calendar resulted in the astronomical Spring Equinox and its Ecclesiastical observation becoming more and more disassociated from each other.  Additionally, “it was found that the paschal moon of the Metonic Cycle was losing all relation to the real paschal moon”⁵ due to its shortcomings.  For these reasons, Pope Gregory XIII (d. A.D. 1585) undertook a reform of the calendar, which not only corrected the shortcomings of the solar Julian Calendar (reassociating the astronomical Spring Equinox with the calendar’s March 21st), but also the independent lunar calendar (reassociating the 14th day of the lunar month with the astronomical Paschal moon).  From this point, the Gregorian March 21st and the Gregorian 14th day of the moon became the Paschal/Ecclesiastical Equinox and Paschal/Ecclesiastical full moon, respectively, used in the determination of the date of Easter (at least for those who accepted the reform).

The Church has never lost track of the fact that she is indeed using two different calendars, one solar and one lunar, to calculate not only the Feast of the Resurrection, but also all the observances which depend on this date – Septuagesima, Ash Wednesday and Lent, the Spring Ember days, the days of Holy Week, the Minor Rogations, the Ascension, Pentecost, the summer Ember Days, Corpus Christi, and the Feast of the Sacred Heart.  In fact, Roman Liturgical books contained detailed explanations, in the section entitled “The Year and its Parts” (De Anno et ejus Partibus) on how to determine Easter, and thus the associated moveable feasts, a calculation which involves, among other things, the Gregorian Golden Number of the lunar calendar.  Additionally, in her daily proclamation of the Roman Martyrology at the Office of Prime, where the entry for the following day is read, the lector begins not only by indicating the entry’s Gregorian calendar day (solar), but also its Gregorian day of the moon, the day of the lunar calendar, with the first day associated with the new moon.  The antiquity of this practice is attested to by Pope Gregory XIII in his decree promulgating his reformed calendar (Inter gravissimas [A.D. 1582], 10).

So now, when you, dear reader, look upon a Catholic calendar which shows the phases of the moon, you can think back to this article and remember the prominent role the moon and its phases play in the shaping of our liturgical year.

Fr. William Rock, FSSP was ordained in the fall of 2019 and is currently assigned to St. Stanislaus Parish in Nashua, NH.

In support of the causes of Blessed Maria Cristina, Queen, and Servant of God Francesco II, King 

1. Old Catholic Encyclopedia, s.v. Jewish Calendar.  See also Bickerman, E. J. (Elias Joseph). Chronology of the Ancient World. (Ithaca: Cornell University Press, 1968), pp. 16-21.
2. See What is the moon phase today? Lunar phases 2024 | Space
3. Old Catholic Encyclopedia, s.v. Epact.
4. Ibid.
5. Ibid.