The “Romulus” calendar was created around 800 BCE. It was based on a luni-solar year with 10 named months. The months that fell during winter were unnamed as they were seen as a dormant period. The calendar year began in March on the Spring Equinox.
In 600 BCE, Roman King, Numa Pompilius named these two dormant months as Inauarius and Februarius. The was reset to begin on the Winter Solstice or December 25th.
In 200 BCE, Inauarius was celebrated as the first month of the revised 12 month calendar. Inauarius is pronounced Januarius, meaning month of Janus.
In 46 BCE Julius Caesar reformed the Roman calendar and choose January 1st as the first day of the year in honor of its namesake Janus. He consolidated the luni-solar Roman calendar system into one solar calendar. He consulted with Alexandrian (Egyptian) astronomer Sosigenes and created a regular year that was divided into 12 equal months. These months were adjusted by adding ten days to the exisitng solar calendar of 355 days, creating a new calendar year of 365 days.
Two extra days were added to the months Ianuarius (January), Sextilis (August) and December, while one extra day was added to Aprilis (April), Iunius (June), September and November. An additional leap day was added to Februarius (February) every four years to account for the fact that a solar year is actually 365.25 days long.
This new calendar was then named the Julian calendar. The Roman senate changed the name of the seventh month of Quintilis to Julius (July) in his honor.
In 8 BCE, Julius Ceasar’s grandnephew, Augustus Ceasar became the emperor of Rome. (he defeated Marc Antony and Cleopatra). The Roman Senate decided that he should have a month named after him. But not only did the Senate name a month after Augustus, but it decided that since Julius’s month, July, had 31 days, Augustus’s month should equal it: under the Julian calendar, the months alternated evenly between 30 and 31 days (with the exception of February with 29), which made August 30 days long. So, instead of August having a mere 30 days, it was lengthened to 31, (they shortened February to 28) preventing anyone from claiming that Emperor Augustus was saddled with an inferior month.
In Roman culture (circa 200 BC – 313 AD) Saturnalia (Saturn is the Roman God of Agriculture) began on December 17th and lasted for seven days culminating on December 25th the Winter Solstice. On that day Mithras the Sun God or Sol Invictus (the invincible sun) was born.
Early Christians didn’t have records of Jesus actual birthday so they chose the Winter Solstice as the perfect day to celebrate it. They saw it as both the rebirth of the “Son” & the “Sun”. Since Jesus taught that the “light” was within each of us, his followers wanted to honor his birth on a day that honored the “light within”. The Winter Solstice was the perfect time to honor that sacred connection.
In 313 AC, the Emperor Constantine saw himself as the Sol Invictus before converting to Christianity. As he converted he declared Christianity as the official “state” religion and officially identified Jesus’ birth as being December 25, the Winter Solstice. Jesus’ birth symbolized the rebirth of the “Son (of God)” versus the “Sun (God)”.
In 1582 CE, the Gregorian Calendar was instituted by Pope Gregory the XIII, as a way to bring order and importance to the “Christian” calendar throughout Europe, Asia and Africa.
As the Conquistadors moved into the Americas during the 1500’s they brought this calendar system with them. It is interesting to remember that people in the Southern Hemisphere celebrate the Summer Solstice which is the opposite of the Winter Solstice.