On this day in 45 B.C., the Julian Calendar took effect in the Roman Empire. This calendar consisted of a solar year of 12 months, 365 days and a leap year every 4 years. However, it is not the one we use today.
Prior to the creation of this calendar, the Roman Empire numbered years from the founding of the city of Rome. By those standards, this day in 45 B.C. would have been 710 AUC (ab urbe condita). However, Julius Caesar wisely determined that this type of calendar would not mean the needs for an empire and tasked the Alexandrian astronomer Sosigenes with creating a new one. The first of January was chosen as the first day of the year since that was the first day the Roman Senate convened under the previous calendar.
Astronomers later realized that Sosigenes’ miscalculations on the length of the solar year caused the Julian Calendar to lose a day every 131 years. No big deal, right? Well, after 16 centuries those lost days were starting to add up. This prompted the creation and adoption of the Gregorian Calendar (named after Pope Gregory) in 1582.This calendar that we still use today corrected the calculation on the solar year and eliminated 10 days from the calendar.
The Catholic countries of Italy, Spain, Portugal and Poland immediately adopted the Gregorian calendar, but it took centuries for the calendar to be fully adopted. The last country to do so was Turkey in 1927.
If you ever want to figure out want to see what today would be on the Julian Calendar, the U.S. Naval Observatory has a calculator for doing so.