Why do so many people ring in the new year on Jan. 1?

After the Thanksgiving leftovers are gobbled and the Christmas trees come down, many turn their attention to the new year, and may celebrate by making resolutions or watching the Times Square ball drop.

Thanks to the widely adopted Gregorian calendar, most people have marked the new year on Jan. 1 for centuries. But with so many other calendar systems — such as the Chinese, Islamic or Hebrew calendar — how did this come to be? And how did it come to represent new beginnings?

How the Gregorian calendar — and Jan. 1 — was born

Let’s go back in time. The Gregorian calendar, and its Jan. 1 start date, has its origins in ancient Rome.

The goal of many early Roman calendars was to find a way to align lunar cycles, solar years and seasons, as several religious festivals and holidays revolved around the equinoxes and moon phases.

For example, many Christians wanted Easter to fall on the spring equinox every year, said Darin Hayton, an associate professor of history at Haverford College.

“So we have a number of competing goals that don’t admit easily of mathematical solutions,” he said.

When a 10-month calendar didn’t do the trick, the Romans borrowed from the Greeks and Egyptians, who figured out that 12 lunar cycles fit into a solar cycle. So Roman King Numa Pompilius extended their calendar to 12 months by adding February and January, named after Janus, the Roman god of beginnings. Jan. 1 was then made the start of the calendar, Hayton said.

In 45 B.C.E., while Julius Caesar was high priest, he stretched the 12-month calendar to 365 days and a quarter of a day. But that quarter amounted to a full day after four years. So he implemented leap years to catch the calendar back up with the solar year, Hayton said.

But there was still a problem. The astronomers of Caesar’s day who calculated the length of a solar year were off by about 11 ½ minutes, a misalignment that would grow significantly over time.

To fix the overcorrection, in 1582, Pope Gregory XIII added a caveat to leap years: a century year (such as the year 2000) will only be considered a leap year if it is divisible by four.

Hence, the Gregorian calendar was born. It was popularized as European countries that were a “dominant economic force” began using it, and took it with them into countries they colonized, Hayton said.

Many cultures use days other than Jan. 1 to ring in the new year

Much of the world uses the Gregorian calendar and its Jan. 1 start date as New Year’s personally and professionally, but many cultures also use their own calendars for social and spiritual occasions.

In some Asian cultures, such as Chinese, Korean and Vietnamese, people celebrate the Lunar New Year between late January and February to coincide with the first new moon on the lunar calendar.

“It emphasizes family reunions, honoring of ancestors, and prosperity,” said Usha Haley, who teaches international business at Wichita State University’s business school.

Rosh Hashanah is the Jewish New Year and falls on the first day of Tishrei, the seventh month of the Hebrew calendar, which is September or October on the Gregorian calendar, according to History.com. Nowruz, the Persian New Year, coincides with the spring equinox in March, Haley said.

Rosh Hashanah “marks a time of reflection, repentance, and spiritual renewal,” Haley said, while Norwuz celebrates “rebirth and nature.”

How to make your own fresh start

People gravitate to New Year’s Day to reset as a part of the “fresh start effect,” said Katherine Milkman, a professor of operations, information and decisions at the University of Pennsylvania’s Wharton business school.

The phenomenon is rooted in humans’ tendency to perceive moments in our lives as chapters, instead of one long continuum, and New Year’s gives us a “chapter break” from old patterns and previous experiences.

“‘That was the old me, and this is the new me, and the new me is going to be different,'” she said. “It gives us optimism about our ability to achieve more.”

New Year’s is the most popular “fresh start” because there’s a strong social pressure, as many other people are doing the same thing at the same time, Milkman said.

But you don’t have to start anew on Jan. 1. A fresh start is still valuable if it’s on your birthday, the first day in a new apartment or even on a random Monday, according to Milkman.

To stick with the goals you’ve laid out for a fresh start, Milkman recommends breaking them down into small, actionable items and making them enjoyable. That could mean partnering with someone on a goal or “temptation bundling,” which pairs your goals with things you like.

“Like, ‘I only get to binge watch my favorite TV shows while I’m exercising,’ or ‘I can only listen to my favorite podcast while I’m cooking a fresh meal for my family,'” she said.

If you prefer to stick to Jan. 1 for your fresh start, don’t worry. Hayton says the Gregorian calendar is unlikely to change anytime soon, as it would be very disruptive. Switching to a new system would likely face heavy resistance due to the potential social costs, like shifting or losing holidays or birthdays.

“The rebel in me would love it to change, but I think that it would take almost an act of God — not an act of the Pope — to get the calendar to change,” he said.