Julian Calendar

In marking the days of the month they went backwards: thus, January first was the first of the kalends of January, December thirty-first was pridie kalendas, or the day next before the kalends of January - the day before that, or the thirtieth of December, tertio kalendas Januarii, or the third day before the kalends of January, and so on to the thirteenth, when came the ides of December.

Julian Calendar - the Names and Origins of the Roman Year
The Roman Year in the Julian Calendar. The founder of Rome, Romulus, instituted a calendar of ten months. The names and origins of the Roman year were as follows:

The first month was called March from Mars, the God or War *** The second April, from the Greek word for the name of Venus *** The third was May, from Maia, the mother of Mercury *** The fourth month was June, from the goddess Juno *** the 5th, July, from Julius Caesar *** 6th, August, from Augustus Caesar *** The remaining months in the Julian Calendar were called from their number - September, October, November, December

Numa Pompilius, the second king of Rome who succeeded Romulus, then added another added two months

January from Janus *** February because the people were then purified, (februabatur) by an expiatory sacrifice which was believed to atone for the the sin of the whole year - this was anciently was the last month in the year.

Julian Calendar - Division of the Roman Year into 12 Lunar Months
Numa, in imitation of the Greeks, divided the year into twelve lunar months, according to the course of the moon, but as this type of division did not correspond with the course of the sun, he ordained that an intercalary month should be added every other year. Julius Caesar afterwards abolished this intercalary month, and with the assistance of Sosigenes, a skilful astronomer from Alexandria. They arranged the year according to the course of the sun, commencing with the first of January, and assigned to each month the number of days, which they still retain. This is the celebrated Julian or solar year on which the Julian Calendar was based, and has been since maintained without any other alteration than that of the new style, introduced by pope Gregory, A. D. 1582, and adopted in England in 1752, when eleven days were dropped between the second and fourteenth of September.

Julian Calendar - Days of the Year
The use of clocks and watches was unknown to the Romans - nor was it till four hundred and forty-seven years after the building of the city, that the sun dial was introduced: about a century later, they first measured time by a water machine, which served by night, as well as by day. The day was either civil or natural day. The civil day was from midnight to midnight whereas the natural day was from the rising to the setting of the sun.

Julian Calendar - Roman Days
The Roman days were distinguished by the names of festi, profesti, and intercisi:

The festi were dedicated to religious worship *** The profesti were allotted to ordinary business ***  The intercisi were the days which served partly for one and partly for the other were called intercisi, or half holy days

Julian Calendar - The Names of the Days
The manner of reckoning by weeks was not introduced until late in the second century of the Christian era. The method of calculating the days of the week was borrowed from the Egyptians. The days were named after the planets:

Sunday from the Sun *** Monday from the Moon *** Tuesday from Mars *** Wednesday from Mercury *** Thursday from Jupiter *** Friday from Venus *** Saturday from Saturn.