Why is New Year's Day on January 1?
The short answer
January 1 became New Year's Day in 45 BCE when Julius Caesar reformed the Roman calendar, aligning it with the start of Roman consuls’ terms. Pope Gregory XIII later cemented this date in 1582 with the Gregorian calendar, now used worldwide.
The long answer
While other calendars mark the New Year with the start of spring, like Chinese/Lunar New Year, or an important religious moment, like Islamic/Hijri New Year, the international-standard calendar begins on January 1.
Why January 1? Well, we can thank the Romans.
Why was January 1 chosen as New Year's Day?
After Julius Caesar became dictator of the Roman Republic in 49 BCE, he decided they needed a new calendar. At the time, the Roman calendar was based on the lunar cycle, with each month lasting about 29.5 days, resulting in a year of only 354 days. This caused the calendar to fall so out of sync with the seasons that they'd need to throw in an extra month to get things back on track – a leap month.
Those in charge of managing the calendar were often politicians, so it was not uncommon for them to abuse their power and lengthen the year when their allies were in control or shorten the year when their opponents were in charge.
To solve this problem, Caesar consulted with astronomer Sosigenes of Alexandria who proposed 365-day solar calendar based on the Egyptian model. In 45 BCE, January 1 was officially designated as the start of this new calendar, known as the Julian calendar.
Bust of the god Janus.
"Museo-etrusco-di-villa-giulia---giano 32493359611 o" by Giuseppe Savo is licensed under CC BY-SA 4.0.
But why January 1? January (Ianuarius) was already a month in the Roman calendar, named after Janus*, the dual-faced god of gates, transitions, and beginnings. Additionally, Roman consuls traditionally began their terms on that date, a practice established in 153 BCE. January 1, already associated with new beginnings, made for a logical start to the new year.
Why did January 1 become widely adopted as New Year's Day?
As Roman armies conquered new lands, the Julian calendar was instituted. This served two purposes: it made calendar-keeping consistent across the empire and it reminded everyone that they were under Roman authority.
But after the Roman empire fell, the practice of celebrating the New Year on January 1 became less consistent. As Christianity spread across Europe, many countries shifted New Year's Day to honor a religious holiday, like December 25 (Christmas Day), March 25 (the Feast of the Annunciation), or whichever day Easter Sunday landed on. However other non-royal, non-clergy people still commonly used January 1 as the beginning of the calendar year. So, in short, there were many New Year's Days for most of European history.
There was one key institution that still used the Julian calendar: the Catholic Church. But the Church discovered a flaw with Caesar's calendar: it overestimated the length of a solar year by 11 minutes. Over time, this would force the Church to keep moving up the Spring Equinox (and, with it, Easter).
"Portrait of Pope Gregory XIII" by Lavinia Fontana is part of the public domain.
In 1582, Pope Gregory XIII had enough and instituted the more precise Gregorian calendar, which held a leap day on years divisible by four, rather than every four years. The calendar also re-established January 1 as the start of the new year.
I'm glossing over several hundred years of history, but, generally speaking, the Gregorian calendar was gradually adopted. Catholic countries switched quickly, but Protestant and Eastern Orthodox regions resisted due to religious concerns. Over time though, most countries accepted it.
Today, even in places where other calendars are culturally significant, the Gregorian calendar is the global standard for civil use – and, with it, New Year's Day being January 1.
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Sources
de Guzman, C. (2023, December 29). Who Decided January 1st Is the New Year?. Time. https://time.com/6550127/new-year-celebration-january-calendar-date-history/
Soniak, M. (2023, December 7). Why Does the New Year Start on January 1?. Mental Floss. https://www.mentalfloss.com/article/29611/why-does-new-year-start-january-1
It’s like an American accent but with calendars.