The Lunar New Year begins on Sunday, and more than a billion people will ring in a fresh year, prompting one of the world's largest annual migrations as observers travel for family reunions.

The holiday is celebrated throughout much of Asia and the Asian diaspora, including among those of Vietnamese, Chinese and Korean descent. The holiday is also celebrated in Mongolia, but in February, as the date is determined with a different calendar system there.

While almost everyone will ring in the Year of the Rabbit in 2023, Vietnam is welcoming the Year of the Cat. Why does Vietnam differ from the rest of the world this year? The origins of the Year of the Cat are murky.

One explanation has to do with linguistics, according to Doan Thanh Loc, a cultural consultant at the Southern Jade Pavilion Cultural Center in Vietnam. It's widely believed that the Chinese word for rabbit sounds like the Vietnamese word for cat, but that's not exactly true.

The date for Vietnam's Lunar New Year, also called Tet Nguyen Dan, is determined using the Chinese lunisolar calendar. Months are set using the orbits of the moon and the Earth, with leap months added every few years to stay in sync with the solar cycle. Each year in the calendar is given a name using a combination of 12 earthly branches — each of which corresponds to an animal in the zodiac — and 10 heavenly stems.

This new year will be named Quy Mao, after the 10th heavenly stem, Quy, and the fourth earthly branch, Mao. In China, the rabbit was chosen to represent the earthly branch called Mao. But in Vietnamese, the pronunciation of Mao can be very similar to how the word "cat" is pronounced. "Mao doesn't necessarily mean cat or rabbit," Doan says. "These are just symbols we've used as code for the earthly branches."

Doan adds that Vietnam hasn't always celebrated the Year of the Cat and that it's unclear when the country switched over from using the rabbit in its zodiac. Mentions of the rabbit in the zodiac appear in many older Vietnamese texts. The uncertainty around the switch between the rabbit and the cat has led to several other theories for its origin.

Quyen Di, a lecturer at UCLA, has several other possible explanations for Vietnam's unique celebration. One has to do with the landscapes of China and Vietnam.

"Originally, the Chinese lived in the savanna area, while the Vietnamese lived in the lowland area," he says. "The people of the savanna prefer a nomadic life, close to the wilderness, and they chose the rabbit as an animal that lived in the wild fields."

In contrast, the lowland people of Vietnam chose the more domestic cat. Additionally, Di says, Vietnamese people consider rabbits as "animals that are used for food" and chose the cat because they're considered "friends living in their house."

Still, these are not the only urban legends surrounding the origin of the Year of the Cat. Ask a Vietnamese auntie or grandparent, and you're sure to hear several more stories about the Year of the Cat.

Many involve the myth of a feast held by either Buddha or the Jade Emperor and a race among the animals to determine their order in the zodiac. In some legends, the cat was disqualified from the zodiac; the rat pushed it into the river. In another, the cat finishes the race and takes its place as the fourth animal.