Friday, February 16, marks the first day of the “Year of the Dog,” according to the Chinese calendar, and for ten days around the new year celebrations — or the Spring Festival, as it’s known in Chinese — families come together for food, fireworks, and gift-giving. It’s like Thanksgiving, Christmas, and New Year’s Eve, all rolled into one. But why is the Chinese New Year celebrated in February, instead of January 1, as in the west?

While the obvious answer might be the moon, it’s actually a little more complicated than that, and the answer is perhaps more accurately, a combination of the moon, the sun, and the seasons.

First, a primer on calendars.

The western, or Gregorian, calendar that has come to dominate the modern world, is based on a solar, or tropical, year, with each other year representing the amount of time between one Spring equinox and the next, which is approximately 365.2422 days (we often forget the .2422 part.)

Moon phases
One complete cycle of the phases of the moon.

The lunar calendar, meanwhile, uses the moon to mark the passage of time, completely ignoring the sun and the seasons as well. In this system, the main passage of time is the lunar month, which measures the amount of time between new moons (29.5 days). The Islamic calendar is lunar, while the Chinese calendar, contrary to popular misconception and the confusingly named “Lunar New Year”, is not.

Instead, the Chinese calendar, which also became the basis of other calendars across Asia, is lunisolar, using the lunar calendar and the solar calendar together to approximate the tropical year. Because there’s an 11-day difference between the ~365 days of the solar year and the 354 days of the lunar year, the Chinese lunisolar calendar adds a leap month every three years to make up for the difference.

So what does that mean for the biggest holiday of the Chinese year?

Chinese astronomers
Chinese astronomers calculating the summer solstice. 

Well, the Chinese New Year is associated with the coming of spring and the start of the planting season, when everything feels new and reborn, so the first day of the lunar new year is typically the first day of the new moon closest to the start of spring. On the Gregorian calendar, that typically translates to a date between January 21 and February 20.

Additionally, because of the 11-day difference in days between the solar and lunar calendars, every year, the Chinese New Year happens eleven days earlier than year before, assuming that it falls within that January 21 and February 20 period. On years with a leap month, it occurs nineteen days later.

So, if you’ve missed the fireworks displays that usually come on Chinese New Year’s Eve, look up anyway — you’ll still likely have a view of the last new moon of winter.