The Julian calendar added ten days and a leap year to the previous calendrical system, bringing the calender into step with the solar year, and established January 1 as the beginning of the year.

In the period preceding Julius Caesar’s rise to power the Romans had used a calendar based on the Greek system, with each year ordinarily consisting of twelve lunar months. Four of these, March, May, July, and October, had thirty-one days each; February had twenty-eight, and the remaining seven had twenty-nine days each. Hence, a combination of the twelve months accounted for 355 days, or 10.25 less than the number of days in the solar year. The Romans were well aware of this discrepancy and had charged the pontifices with the responsibility of taking care of it by inserting intercalary months from time to time in order to keep the calendar in tune with the seasons. For special reasons of their own, however, the pontifices often failed to insert intercalary months when they were needed, with the result that months that were supposed to be winter months eventually fell in some other season. In the year 46 b.c.e., because of the earlier failure of Julius Caesar himself to declare intercalary months in his capacity as pontifex maximus, the calendar was found to be sixty-seven days behind the true date as indicated by the position of the sun. Caesar, Julius
Sosigenes

Caesar, with the help and advice of the Alexandrian astronomer Sosigenes, set out to rectify this situation. In addition to proclaiming a regular intercalary month for 46 b.c.e., he inserted two additional months between the last of November and the first of December, thereby adding sixty-seven more days to that year. Consequently, January 1 of the following year, 45 b.c.e., corresponded to the solar January 1, or kalends of January as the Romans named the day.

To remove the need for inserting intercalary months in the future, Caesar lengthened some of the months by one or two days. January, August, and December were given two additional days each, being changed from twenty-nine to their present thirty-one. April, June, September, and November each acquired one additional day, being changed from twenty-nine to the present thirty. These two sets of changes accounted for the addition of ten days. February retained its former quota of twenty-eight days, but the one-fourth day left over was accommodated by the inclusion of an extra day in February every fourth year in anticipation of our present leap-year arrangement.

This new Julian calendar was so nearly accurate in its measurement of the solar year that only one minor revision was required in the interval between antiquity and modern times to bring it up to date. The year is actually a little less than three hundred and sixty-five and one fourth days in length. To rectify this minor error, Pope Gregory XIII in 1582 introduced a plan of ignoring leap-year once every four hundred years.

Caesar retained, for the time being, the system that had prevailed in earlier periods whereby days of the month were designated as falling on or a certain number of days before the kalends, ides, or nones of a given month. The kalends are in all cases the first of the month, but in the case of March, May, July, and October, the nones fall on the seventh and the ides on the fifteenth, while in the other months the nones fall on the fifth and the ides on the thirteenth. January 12 is ordinarily designated as the day before the ides of January, but January 11, because of the inclusive system of counting used by the Romans, is designated as the third day before the ides of January. The names of the months as used by the Romans are actually adjectives which modify the forms of the words kalendae, nonae, and idus used in the formulae. In most cases, the Latin names of the months can readily be recognized from their similarity to the English names that are derived from them. It should be noted, however, that July was originally called quinctilis and August sextilis, the fifth and sixth months in a system in which the year began with March rather than with January. At a later date the names July and August were established in honor of Julius Caesar and Augustus Caesar (63 b.c.e.-14 c.e.), respectively.

In converting Roman dates listed as preceding the nones or ides of a given month, one uses the same rules for both the pre-Julian and Julian calendars. Because a date preceding the kalends, however, falls in the preceding month, it is necessary to take into account the number of days in that month in making the conversion. This procedure in turn means that in such cases a careful distinction must be made between the pre-Julian and Julian calendars, since the number of days for several months differs in the two cases.

A calendar is an indispensable device to measure time so that people may regulate their activities. Societies have sought from earliest times to erect a system of time based on the alternation of day and night, the rising and setting of the sun, phases of the moon, and even on the change in position of stellar constellations. Greek astronomers after the fifth century b.c.e. tried constantly to calculate the solar year more accurately. Caesar’s fame in respect to the calendar rests not only on the fact that he availed himself of the latest Hellenistic knowledge to adjust the year but also on the fact that he had his system adopted as the official calendar of Rome.

Because much Roman work on the calendar has been lost, Ovid’s Fasti (c. 8 c.e.; English translation, 1859) and certain inscriptions are invaluable. Intricacies, uncertainties, and contradictions abound in the study. It is clear enough that Rome started out with a lunar reckoning and changed to a luni-solar calendar which kept the lunar months as basic units for the year but adjusted the year to sun time by inserting intercalary months. This pre-Julian calendar was adopted by the Decemviri probably in 451 b.c.e. in connection with the Twelve Tables. In a cycle of four years, each with 355 days, or 1,420 days in all, 22 or 23 days were added to the second and fourth year. This arrangement closely approximated the solar year. The pontiffs, charged with the task of intercalation, often manipulated these additions to benefit or injure politicians and contractors.

If most calendars have religious implications so that it takes a caesar or a pope, or a French or Russian revolution, to change them, the Roman calendar was by its very nature religious. If without the approval of the gods no law could be passed, it was to be expected that all activities of the state must be carried out when the gods approve. Consequently, it was natural that the colleges of priests as official bureaus charged with divining the will of the gods should control the calendar. It has to be accepted that everything that the Roman calendar tried to regulate, whether actions at law, meetings of the assembly, market days, or festal days, was associated with religion.

All days of the calendar were religiously designated to mark their distinct character. Certain days were clearly listed as dies nefasti when actions at law or civil and legislative functions could not be conducted without risking divine displeasure. Some days were nefasti in the morning, apparently when the sacrifice was being immolated, and in the evening when the final offering of the victim was made. The period in between was fastus. Of the 355 days of the Republican year, the character of only 48 is uncertain. Dies fasti, when action at law was permitted, seem to have occurred regularly on the same dates each month. Only forty-two are listed for the year, obviously less than the demand for justice would require. Clearly other days were used for legal affairs. Since only that legal action in which the praetor played a part was forbidden on dies nefasti, action probably went on almost any other day in individual courts to which he referred cases. The largest group of days designated in the calendar allowed the comitia, the assembly of the people, to meet, although these days might be changed to a nonbusiness status if priests or magistrates announced special times to expiate prodigies or to engage in thanksgiving or supplication. The rules by which the character of comitial days could be altered were naturally used to the advantage of politicians.

Probably the dies feriati show best the basic religious nature of the Roman calendar. In the late Republic, these days having lost much of the original meaning, purpose, and activity became simply optional holidays of rest when courts were shut down and certain agricultural work was restricted. The fact that dies feriati could show a cheerful or a serious mood points to a religious origin; apparently they were days believed to be defiled by work. Since the basic pattern of the Roman calendar was no doubt dictated by rites connected with seasonal activities dealing with crops and flocks, plowing and sowing, ferial days were of agricultural origin. Because weather is important in this context, dies feriati were movable. As the state grew more urban and citizenship became wider, the feriae took on a vague religious character for the whole state and were given fixed dates. In time they came to be connected with legendary or historical events so that in 45 b.c.e. the senate honored Julius Caesar by establishing feriae on the anniversary of his victories. Caesar changed little. The division of the months into ides, nones, and kalends is of lunar origin. Moreover the civic-religious designation of each day was retained even though much of the meaning had been lost. The old designations were most likely kept to make it easier to remember the calendar.

• Duncan, David E. Calendar: Humanity’s Epic Struggle to Determine a True and Accurate Year. New York: Bard, 1998. This popular overview history discusses calendars and time-keeping throughout world history.
• Richards, E. G. Mapping Time: The Calendar and Its History. New York: Oxford University Press, 2000. A very broad-based history of time-keeping, with a chapter on the Julian calendar.
• Steel, Duncan. Marking Time: The Epic Quest to Invent the Perfect Calendar. New York: John Wiley and Sons, 2000. One chapter is devoted to Julius Caesar’s reformation of the Roman calendar.

Julius Caesar. Julian calendar