The Julian calendar was introduced by Julius Ceasar in 45 BCE in response to the confusion of the previous Roman calendar which was very inaccurate and required regular meetings of officials to decide when days should be added or removed to keep the calendar aligned with the seasons. To further the confusion, it seems these officials sometimes added or removed days to suit their own ends, for example, they might remove a few days to get a public official they were not fond of out of office quicker.
The Julian Calendar consisted of 365.25 days, with a leap year of 366.25 days, every 4 years. The leap day fell on February 24th. This calendar introduced an extra day every 128 years and this was corrected by the introduction of the Gregorian calendar in 1582.
The new calendar kept significant days of the month that were originally identified under a more ancient lunar system that began with the first appearance of the new moon crescent.
Kalends- the first day of the month, sacred to Juno.
The Nones originally marked the first quarter moon and then fell on the 7th day of months with 31 days and the 5th on all other months.
The Ides originally occurred at the full moon and later halfway through the month; on the 15th during months with 31 days, and the 13th of all other months.
Dates were counted backward from these days. For example, you would say your birthday was 3 days before the ides of September, instead of September 10th.
Martius – March – Month of Mars
Aprilis – April
Maius – May
Iunius – June – Month of Juno
Quintilis – “fifth month” Changed to Iulius (July) in honor of Julius Caesar
Sextilis = “sixth month” Changed to Augustus (August)in honor of Caesar Augustus
September – “seventh month”
October – “eighth month”
November – “ninth month”
December – “tenth month”
Ianuarius – January – Month of Janus
Februarius – February
Mercedonius/Intercalaris – this month existed just for extra days when needed prior to the creation of the Julian calendar. It was removed when the new calendar was developed.