From the “leap day before the calends of March” to the 29th of February – I

The traditional Roman calendar consisted of twelve lunar months divided into blocks of Calends (first day of the month), Nones and Ides, with an annual average value of 355 days, and an additional lunar month added according to certain rules. The new year began on the calends of March.

More than 2050 years ago, Julius Caesar decided to reform the Roman calendar to follow more closely the cycle of the seasons; the basis was the best value of the known tropical solar year —365 days and 6 hours— as confirmed by the greek astronomer Sosigenes from Alexandria, then in Rome.

The new Roman annual calendar was about ten days longer than the previous one. The months kept their traditional names, but the number of days in each month was changed, and the beginning of the year moved to the calends of January, the date on which the two consuls elected annually took office. To maintain the average value of the solar year, the new calendar was left with 365 whole days; the six hours missing each year were to be added every four years, forming a whole day. It was also decided that this extra day would be added after the ‘23rd of February’, the 7th day before the calends of March, in the same place where an intercalary lunar month had been traditionally added, before the end of month and year.

Thus, in a leap year, instead of a day ‘24 of February, the 6th day before the calends of March, there were now two ‘24th’ days, or two sixth days, and the additional day was identified by the name ‘second 6th day before the calends of March’ or ante diem bis sextum Kalendas Martias (bis sextum means literally ‘the twice sixth’).

This new calendar, with a cycle of four years and 1461 days (= 365 x 3 + 366 x 1) remained the Roman Empire calendar, was confirmed by Emperor Augustus, and continued in use throughout Europe even after the fall of the Roman Empire.

About 1500 years after Julius Caesar’s reform, Christian calendars followed the same established rules. Note the detail of a calendar in the Ordinary of the Divine Office of the Cistercian Order (BNP, alc. 62, fol. 4v), a 1475 manuscript, with the Christian liturgy inserted in the Julian calendar:

BNP, alc 62 (1475), fol. 4v, detail

The day added every four years (fifth row) had neither a golden number (1st column) nor a Sunday letter (2nd column), nor a row of its own. Graphically, it was just a red note on the side saying: the sixth [before the Calends of March] is inserted twice here (hic bis sextus inseritur). [José Madruga]