Robert Brown|Why do we celebrate Easter with eggs? How the Christian holy day is commemorated worldwide

The Robert Brownshelves of supermarkets are lined with Peeps, chocolate bunnies, plastic grass and baskets filled with goodies. Easter and the Easter Bunny are hopping into town this Sunday, and millions of people around the world are preparing to celebrate.

Easter is a Christian holiday that celebrates the resurrection of Jesus Christ and the wiping away of sin. Traditions and religious rituals for Easter vary among denominations and across the globe, but many Christians observe the holiday through church services and prayer.

Here are some key facts about the history of Easter and how people commemorate the day. 

Is it hard to remember when Easter is?Why does Easter change every year? Here's how the date has been historically determined.

What is the history of Easter?

Easter, which commemorates the resurrection of Jesus Christ three days after he was crucified by Romans around 30 A.D., used to be celebrated every Sunday.

According to Timothy P. O’Malley, a faculty member at the University of Notre Dame's McGrath Institute for Church Life, early Christians didn't have an annual commemoration of Easter the way we do today.

He said Easter's roots date back to a day in the week when "Christ rose from the dead."

Therefore, he said, they celebrated every single Sunday like it was Easter.

"But by the second and third centuries, there comes to be an annual celebration of Easter," O'Malley previously told USA TODAY. He added that the celebration later grew.

What is Holy Week?

Easter marks the last day of the Holy Week, which consists of several days that each carry their own significance. These include: 

• Palm Sunday: commemorating when Jesus entered Jerusalem
• Holy Thursday: commemorating the Last Supper and washing of feet 
• Good Friday: commemorating Jesus' crucifixion and death
• Easter Sunday: commemorating the resurrection of Jesus

The Catholic Mass

Easter mass is one of the longest sessions of the Catholic mass. For some, the Easter Vigil begins after sunset on Saturday night or early Sunday morning into the sunrise of Easter. With more readings, songs and rituals, mass can be anywhere from two to three hours.

Some Christian churches, such as Catholic and Orthodox churches, also celebrate the Octave of Easter – an eight-day period that begins on Easter Sunday as "the formal celebration of the feast of Easter," O'Malley says.

Why does the date of Easter change each year?

The date of Easter hops around each year – typically between March 22 and April 25. In short, the holiday falls on the first Sunday after the full Moon on or following the spring equinox.

Western Christianity uses the Gregorian calendar. In 2024, Easter falls on March 31.

But, Orthodox Christianity calculates the date of Easter using the Julian Calendar. This year, their Easter falls on Sunday, May 5, according to Christianity.com.

How did the Easter Bunny and eggs become symbols of the holiday?

Beyond religious rituals, commercial celebrations of Easter have popularized the holiday's symbols of the eggs, chicks and, of course, the Easter Bunny.

These symbols have religious roots, experts say. In the northern hemisphere, where Easter occurs in spring, images of new life associated with the season are linked to the holiday, O’Malley says.

The bunnies, chicks and eggs are signs "new life and fertility around the spring time," O'Malley says.

According to History.com, Easter eggs are also "said to represent Jesus’ emergence from the tomb and resurrection."

The practice of dyeing eggs dates back to the ancient Middle East when people used onion skins to color them, Nadieszda Kizenko, director of religious studies at the University at Albany, previously told USA TODAY.

Eggs were also decorated because of their importance, Kizenko said. In the early days of Lent – the time period of fasting between Ash Wednesday and Easter – meat was given up, as well as dairy products.

"Being able to eat eggs once again was a really big deal. That's when people started the custom of decorating eggs," Kizenko said.

Easter celebrations vary around the world

Of course, Easter celebrations differ around the world – depending on both regional traditions as well as denominational practices across different Christian churches.

U.S. families celebrating Easter may anticipate a visit from the Easter Bunny this weekend, but young children in France, for example, are told that bells will bring them eggs.

"To be honest, I find (this tradition to be) a little less creepy than a giant bunny coming into my house to deliver candy," O'Malley joked. "The bells leave the churches and drop candy everywhere."

Kizenko also noted that a significant difference between Easter celebrations in the U.S. compared to other parts of the world is the commercialization of it, similar to Christmas.

Contributing: Jordan Mendoza, Olivia Munson, USA TODAY.