The Julian calendar takes effect for the first time on New Year’s Day

On January 1, 45 BC, the Julian calendar took effect for the first time. From the article:

"The Julian calendar takes effect for the first time on New Year’s Day
In 45 B.C., New Year’s Day is celebrated on January 1 for the first time in history as the Julian calendar takes effect.

Soon after becoming Roman dictator, Julius Caesar decided that the traditional Roman calendar was in dire need of reform. Introduced around the seventh century B.C., the Roman calendar attempted to follow the lunar cycle but frequently fell out of phase with the seasons and had to be corrected. In addition, the pontifices, the Roman body charged with overseeing the calendar, often abused its authority by adding days to extend political terms or interfere with elections.

In designing his new calendar, Caesar enlisted the aid of Sosigenes, an Alexandrian astronomer, who advised him to do away with the lunar cycle entirely and follow the solar year, as did the Egyptians. The year was calculated to be 365 and 1/4 days, and Caesar added 67 days to 46 B.C., making 45 B.C. begin on January 1, rather than in March. He also decreed that every four years a day be added to February, thus theoretically keeping his calendar from falling out of step. Shortly after Caesar was assassinated in 44 B.C., Mark Anthony changed the name of the month Quintilis to Julius (July) to honor him. Later, the month of Sextilis was renamed Augustus (August) after his successor.

Celebration of New Year’s Day in January fell out of practice during the Middle Ages, and even those who strictly adhered to the Julian calendar did not observe the New Year exactly on January 1. The reason for the latter was that Caesar and Sosigenes failed to calculate the correct value for the solar year as 365.242199 days, not 365.25 days. Thus, an 11-minute-a-year error added seven days by the year 1000, and 10 days by the mid-15th century.

The Church became aware of this problem, and in the 1570s Pope Gregory XIII commissioned Jesuit astronomer Christopher Clavius to come up with a new calendar. In 1582, the Gregorian calendar was implemented, omitting 10 days for that year and establishing the new rule that only one of every four centennial years should be a leap year. Since then, people around the world have gathered en masse on January 1 to celebrate the precise arrival of the New Year."