First day of spring arrives: 5 things to know about the vernal equinox

It's the moment warm weather fans have been waiting for: the first day of spring has officially arrived in the Northern Hemisphere, though some states may not believe it.

A winter weather storm system is expected to creep up the Middle Atlantic and Northeast coasts Tuesday night and Wednesday, and it's bringing a lot of snow.

"Expect mixed precipitation for the Mid-Atlantic on Tuesday, switching to snow on Wednesday from the Mid-Atlantic to New England," the National Weather Service (NWS) warned on Tuesday. 

The powerful storm could dump up to 18 inches of snow in some locations, marking the fourth nor'easter to smack the region in three weeks.

Winter weather may still be lingering, but that hasn't stopped people across the country from celebrating what's known as the vernal equinox.

Here are 5 things you should know about the annual March event.

What is the vernal equinox, and why do we have it?

Equinoxes occur twice a year, in March and September, to mark the onset of spring and autumn. During an equinox, which in Latin translates to "equal night," both day and night are equal.

"Today the length of night and day are nearly equal," the National Oceanic and Atmospheric Administration (NOAA) said early Tuesday. "The days will now become longer at the higher latitudes because it takes the sun longer to rise and set."

On this day, the sun crosses the celestial equator – "the imaginary line in the sky above the Earth’s equator – from south to north," the National Weather Service in Jackson, Mississippi explained in a tweet.

When does the equinox officially occur?

The sun will be directly overhead at approximately 12:15 p.m. ET on Tuesday, according to Time and Date.

Does the vernal equinox fall on the same day each year?

No. The first day of spring can arrive anywhere from March 19 to March 21, depending on the year. 

"Due to time zone differences, the equinox may occur a day earlier at locations that are behind UTC," Time and Date adds.

Why? Our calendar year doesn't always have an even number of days. Every four years, an extra day, known as Leap Day, is added in the month of February.

"The March equinox would occur on the same day every year if the Earth took exactly 365 days to make a complete revolution around the Sun. But this is not the case," Time and Date explains. "It takes the Earth about 365.25 days on average to go around the Sun once."

Why do people try to balance eggs on this day?

An ancient myth claims an egg can balance on its end only during a vernal equinox. And every year, people gather together to attempt the challenge.

"The myth was popularized in the United States following a LIFE article in 1945, which explained the old spring adage," AccuWeather reports.

But that myth has proven to be false.

"The vernal equinox brings no special egg-balancing properties with it," fact-checking website Snopes.com confirmed in a post online. "Standing an egg on its end is something just about anyone can do any day of the year; the feat simply takes the right egg and a little trial and practice."

Why do people flock to Chichén Itzá on this day?

During the equinox, people often turn to Chichén Itzá, an ancient complex constructed by Mayans located in central Mexico, to watch the sunset. They're hoping to witness a very special shadow that's cast on the structure.

"Incredibly, twice a year on the spring and autumn equinoxes, a shadow falls on the pyramid in the shape of a serpent," National Geographic states. "As the sun sets, this shadowy snake descends the steps to eventually join a stone serpent head at the base of the great staircase up the pyramid’s side."

The Associated Press contributed to this report.