© Getty Images

How it all started -

In 1582, Pope Gregory XIII introduced the Gregorian calendar, which is the calendar Catholics and Protestants use to celebrate Christmas on December 25.

© Getty Images

The Gregorian Reform -

The reason for the Gregorian Reform was to make sure that Easter, the Church's most important festival, always fell around the spring equinox in response to a gradual time drift that was occurring under the Julian calendar.

© Getty Images

The use of the Julian calendar -

However, Orthodox Christians, excluding the Greek Orthodox Church, which adopted the Gregorian calendar in 1923, and Ukraine, still use the Julian calendar to this day.

© Getty Images

Julian calendar -

The Julian calendar is a solar calendar that was implemented in 46 BCE by Julius Caesar. It's a whole 13 days later than its Gregorian counterpart.

© Getty Images

A miscalculation -

The almost two-week gap was caused by a slight miscalculation when the Julian calendar was first implemented.

© Getty Images

A greater gap -

This miscalculation is actually leading the gap to fall more and more out of sync with the solar year as the centuries pass.

© Getty Images

A new date for Orthodox Christmas -

As a result, Orthodox Christmas will eventually fall on January 8 by the year 2100.

© Getty Images

Orthodox Christmas today -

For now, starting on January 6, Orthodox Christians, both in Eastern Europe and Ethiopia, traditionally fast until the first star appears in the night sky, symbolizing the birth of Jesus.

© Getty Images

Traditional greetings -

It is traditional to greet friends and family with "Christ is born!" and reply with "Glory to him!" or "He truly is born!"

© Getty Images

Not to be confused with the day of the Epiphany -

The January 6 celebrations are not to be confused with the day of the Epiphany, which is celebrated by Christians following the Gregorian calendar to mark the physical manifestation of Jesus.

© Getty Images

Day the three wise men appeared -

January 6 is also the day the three wise men appeared in Bethlehem following Jesus' birth. This is most notably celebrated in Spain as El día de Los Reyes Magos.

© Getty Images

Orthodox Christmas Eve meal -

The Orthodox Christmas Eve meal is traditionally meat- and alcohol-free, usually consisting of 12 dishes representing the 12 apostles.

© Shutterstock

Christmas porridge -

In some parts of the Orthodox world, one of the staple foods is kutia, a cold porridge-like dish made out of whole wheat, poppy seeds, raisins, walnuts, and honey.

© Shutterstock

Christmas Day -

Christmas Day on January 7 starts with a visit to church. Some Orthodox Christians prefer to go to a midnight mass on Christmas Eve.

© Getty Images

Midnight mass -

The All-Night Vigil involves the celebration of Matins, the hour which is traditionally observed at midnight.

© Getty Images

No gifts on Christmas Day -

Contrary to Western Christmases, it's not customary to give presents in the Orthodox tradition.

© Shutterstock

But there are gifts on other days -

However, Orthodox Christians usually hand out gifts on December 19, Saint Nicholas Day, or on New Year's Eve on December 31.

© Getty Images

New Year's Eve gift exchange -

In Orthodox Christian countries in Europe, the custom of exchanging gifts on New Year's Eve stems from the communist era. Back then, authorities believed a more secular holiday would take precedence over a religious one.

© Getty Images

Julian calendar for the festivities -

While Orthodox Christian countries in Europe use the Gregorian calendar, the faithful often revert to the Julian calendar for the religious festivities.

© Getty Images

The Julian calendar New Year -

According to the Julian calendar, the New Year is between January 13 and 14, with the day of the Epiphany taking place on January 19.

© Getty Images

An Epiphany tradition -

In countries that adopted the Gregorian calendar, like Bulgaria and Greece, priests will often throw crucifixes in rivers and lakes on the day of the Epiphany.

© Getty Images

Dive for the cross -

Then, men will dive into the cold water to retrieve the cross. The ceremony is meant to symbolize Jesus' baptism.

© Getty Images

Orthodox Christmas in Ethiopia -

Christmas is celebrated on January 7 in Ethiopia, with women wearing a thin white cotton garment called a netela. On Christmas Eve, Ethiopian Christians attend an overnight church service, usually starting around 6 pm and finishing at 3 am.

© Getty Images

Ethiopian pilgrimages -

In Ethiopia, people line up surrounding a church and begin a long, looping procession around it. The holiday attracts pilgrimages to the 11 Lalibela Churches.

© Getty Images

Orthodox Christianity in the Middle East -

Orthodox Christianity is the most common form of Christianity in the Middle East, with a majority living in Egypt and the Levant. The night before Christmas Day, on January 6, a church service takes place until midnight. After the service, people share a specific type of bread called qurban. The bread is marked with 12 dots, symbolizing the 12 apostles.

© Getty Images

Christmas Eve morning in the Western Balkans -

Across the Western Balkans, predominantly amongst Serbians, Orthodox Christians mark Christmas Eve morning by going to the woods to chop down part of an oak tree, commonly referred to as a badnjak.

© Getty Images

Badnjak -

The badnjak is then brought home, with most of it set on fire. A branch with leaves is used for decoration.

© Getty Images

Ukrainian street performance -

In Ukraine, the period between January 6 and 19 is a time for vertep, a puppet theater street performance based on the nativity.

© Getty Images

Vertep -

Vertep is accompanied by singing and dancing, and has links to pagan, pre-Christian traditions.

Sources: (Euronews) (CNN) (Time) 

See also: Traditional Christmas dishes from around the world

© Getty Images