Table of Contents

• Introduction
• Lunisolar cycles
• Old Roman Calendar
• Julian calendar
• The Council of Nicaea in 325
• The Roman Dating System
• Calendar eras
• Different beginnings for the new year
• The Martyrology
• Dies caniculares
• Dies egyptiaci
• Gregorian calendar reform
• Reception of the Gregorian Calendar
• When Easter is celebrated
• Calculation of Easter Sunday in the Julian calendar
• Calculation of Orthodox Easter Sunday in the Julian calendar
• Calculation of Easter Sunday in the Gregorian calendar
• Calculation of Easter Sunday online
• Calendarium gregorianum
• Table of dates for the Easter holidays
• Distribution of Easter Sundays in the Gregorian Calendar
• Graph distribution of Easter Sunday depending on the epact
• The difference between Easter Sunday in the Julian and Gregorian calendars
• The Easter Paradox
• Chronological Cycles
• Golden number
• Epacts
• Solar cycle
• Dominical letter
• Indiction
• Gaussian algorithm for calculating Easter Sunday
• Carter's algorithm for calculating Easter Sunday
• Tabula Paschalis Nova Reformata
• Tabula Noviluniorum
• French Republican calendar

The Roman Dating System

Kalendae, Nonae, Idus and their corresponding lunar phases

The Romans counted the days of each month in a somewhat confusing manner. This stemmed from the fact that the ancient Roman calendar was, above all, interested in lunar observations. In order to respect the changes that occurred between each phase of the moon, each month was given three fixed days with their own name. The first day of the month was called the kalends (kalendae in Latin; this likely explains the origin of the word “calendar” in English). On this day, the pontiff declared the start of the new moon and the cycle was given an official start date. The next fixed day was the nones (Nonae in Latin), which was announced on the first quarter (by which they meant the first “phase”). The third and last fixed day was the ides (Idus in Latin), or what is known to us today as a full moon. You may be wondering what happened to the last quarter: the Romans simply ignored this last phase of transition where the moon reverts back to its new moon phase. For months that originally possessed 31 days (Martius, Maius, Quintilis - later Iulius and October), the nones was set as the 7th day and the ides was on the 15th. For the rest of the months of the year, the nones was established as the 5th day while the ides moved to the 13th. After later changes to the calendar were made, it no longer became possible to keep the calendar and the moon phases in check. Not wanting to remove the Kalendae, Nonae, and Idus, accommodations were made to retain these fixed dates.

To understand these adjustments, first one needs to be aware of a couple features in the Latin language. The first thing to know is that their prepositions fall under two categories: accusative and ablative. The second thing to know is that beyond the surface understanding of each preposition, these two categories convey a sense of moving "towards/forward" (accusative) or "away/backwards" (ablative). For example, while the Latin preposition post may functionally be used to describe something that comes "after" something else, because it's an accusative preposition, it also conveys a positive, forward-moving sense. So, when describing a date that occurred one day before the kalends, nones, or ides, the Romans opted for the accusative pridie (it was not common for them to describe dates from the perspective of after these fixed dates, but when they did, they used the accusative postridia). The rest of the days of the month were not given a name. Instead, they would simply count the number of days until the next fixed date, and preface that number with the accusative ante diem (“before the day”). It should be noted that kalends, nones, and ides are always used in the plural in English as it is in Latin (this is why we add “the” before using any of them in a sentence, such as in “the ides of March”).

Some examples:

• Kalendis Ianuariis (ablative, “the kalends of January”) is the 1st of January
• Nonis Februariis (ablative, “the nones of Februrary”) is the 5th of February
• pridie Idus Martias (accusative, “the day before the ides of March”) is the 14th of March
• ante diem VI Nonas Iulias (accuasitve, “six days before the nones of July”) is the 2nd of July
• a.d. XII Kalendas Aprilis (accusative, “12 days till the kalends of April”) is the 21st of March

Conversion Table:

For the end of February, the table on the left shows the names of the days for a possible leap year. You can see that a.d. VI Kalendas Martias (six days before the kalends of March) occurred twice in successive years, and because of that, the designation for the following days did not have to change. Hence, the later term Dies Bissextilis, or “twice the sixth day” (before March). In the Middle Ages, this was used for the name of the entire leap year - annus bissextilis. Unfortunately, it is no longer possible to determine exactly whether this day was on the 24th or the 25th of February. That said, the 24th of February is more likely, as it was the date later used by the Church. Then again, in Roman law, the 25th of February was considered a leap day.

Later on in the Middle Ages, the classical form of denoting days was abandoned, as the names of days became normalised, and were considered nouns in their own right that did not need conjugation. For example, dates came to be written as Kalendas Iulii instead of Kalendis Iuliis.

The Roman calendar was originally divided into eight-day weeks. The individual days were marked with capital letters from A to H, while A was a day of rest. The Romans began to use the seven-day week at the beginning of the 3rd century AD, and its days were named after the gods to whom they were consecrated.