The medieval calendar was based on the Roman tradition of the Julian calendar. The Julian calendar used 12 lunar months to track one solar year, with irregular days spread out to keep it as even as possible. Medieval calendars tended to use modified Latin spellings: Januarius, Martius, Maius, Julius, Augustus, October, and December had 31 days. April, Junius, September, and November had 30 days, while Februarius had 28. Februarius had an extra day in leap years, as in modern times.
Charlemagne tried to introduce a new calendar of months using the Franks’ native language. His year began with renaming January as Wintarmanoth, and next was Hornung. The months then used the significant events and farm occupations of each stage in the seasons: Lentzinmanoth, Ostermanoth, Winnemanoth, Brachmanoth, Heuvimanoth, Aranmanoth, Witumanoth, Windumemanoth, Herbistmanoth, and Heilagmanoth (Holy Month, because of Christmas). The calendar reform did not last beyond Charlemagne’s time and was never widespread. The church, centered in Rome, used the Roman calendar, so the Franks adopted it.
Traditionally, people tracked significant dates by the liturgical calendar. A baby was born just before Martinmas; a fair was held on Saint John’s Day; pilgrims would set out on Whitsun or Midsummer Day. Knowing these saints’ days in order was a daily-life skill. The best-known ones were as well-known as modern Thanksgiving, but lesser saints’ days could be memorized in a poem designed to put them in order in a catchy way.
The modern system of counting numbered days through each month developed during the Middle Ages for business and record keeping. Bede used simple numbers in his history of the English people. By the 14th century, notaries simply wrote down the numbered day of the month. However, in the books of hours, and officially, Europe followed the old Roman system of numbering days. It was a complicated method, but it had the virtue of tracking the phases of the moon, which helped in calculating lunar-determined feasts.
Romans divided their months into periods based on the phase of the moon. There were 3 key days. Calends (or kalends) was the 1st day of the month, the new moon. It was called calends because a Roman official had called out (Latin calere) the start of a new month and time to pay interest on debts. Ides meant the middle of the month, the time of the full moon, usually the 13th or 15th day. Nones was 9 days before the full moon. Only these 3 dates were distinguished, and all other days were named in relation to them. If the ides of a month was the 15th day, then the 14th day was the “pridie idus”—the day before the ides. Each day was named by working backward to the next key day.
The medieval calendar used the Roman system. The month’s first day was Calends, and the next day, depending on the month, either IV or VI before Nones, and on with a countdown to the Pridie Nonus and Nones itself. There was then a countdown to the Ides: VIII, VII, VI, V, IV, III, Pridie Idus, and Ides. The second half of the month counted down to the Calends of the next month. Holy Innocents’ Day, which we call December 28, was “ante diem IV Januariis.” On the chart in a book of hours, one column tracked these countdowns.
Although there was an understanding that the first of January began a Roman new year, most medieval societies regarded a date in spring as the beginning of the new year. In England and some other European countries, the year began on the Feast of the Annunciation, March 25. In France, the year began on Easter, although the date moved. Some people considered the year to begin at Christmas. Only in the 14th century, as international banking required standardization of dates, was the first of January firmly established as the start of a year. Traditional celebrations continued to be held in spring until the 18th century.
One of the most contentious issues in the early Middle Ages was how to calculate the day of Easter. In England, the Irish tradition of Saint Patrick was not the same as the Roman tradition of the later-converted Anglo-Saxons. Some English monasteries had been founded by Irish monks and continued to calculate Easter the Irish way. In 664, there was a national conference, the Synod of Whitby, on how to decide when Easter would fall. They voted to adopt the Easter tables drawn up by Dionysius Exiguus in 525 in Rome.
Dionysius (or Dennis) tackled the difficult problem of how to name years, as well as how to calculate Easter. The Roman Empire had named years for the emperor or consul, a system that ceased to work well after the fall of the empire in the West. Dionysius, a monk who lived in Rome in the sixth century, drew up a table showing the dates of Easter in many years. He identified that he was writing it 525 years after the birth of Christ, or Anno Domini—“in the year of our lord.” He used this approximation to rename the years that had been named for Emperor Diocletian, who persecuted Christians. There was no way to check Dionysius’s statement of when Jesus had been born, but it was based on church traditions and calendar systems. Although he did not recognize the need to name the year of Christ’s birth the Year Zero, his tables made the first use of the concept of zero in a Latin work. Calculation of Easter was based on a cycle of 19 years, and he designated the beginning of each period as nulla, the Year Zero for each cycle.
The English monk Bede studied Dionysius’s tables and extended them into the eighth century following the Synod of Whitby. He wrote several books on how to calculate time, and he also wrote The Ecclesiastical History of the English People. He dated his work 731, according to the new Anno Domini system of Dionysius. Bede’s book was more influential than he might have imagined. It was copied many times and came to the continent when Charlemagne recruited the Anglo-Saxon scholar Alcuin to found his school and library. Through Bede’s history and Dionysius’s Easter tables, the Anno Domini dating system spread.
During the Middle Ages, scholars began to notice that the Julian calendar was not precise enough. Over the years, the new and full moons no longer matched the Calends and Ides as they were supposed to. Every year there were about 11 minutes unaccounted for. By the ninth century, astronomers were noticing the discrepancy. In the 13th century, Roger Bacon could see the problem clearly and criticized the Julian calendar. Pope Clement VI received a report suggesting that four days should be dropped in 1349, but the chaos of the plague of 1347–1348 buried this recommendation. In 1436, a Council at Basle received a report by Nicholas of Cusa that recommended that they drop out one week between Easter and Pentecost. The council felt it would be too confusing and that it would interfere with merchants’ calculations of interest on loans. The corrected Gregorian calendar was not adopted until 1582, and it was not until 1752 that days were deleted to correct the growing inaccuracy.