The first full day of Lent for churches using the "old calendar" to calculate the date of Easter is Monday, March 18. 

In the Eastern Christian tradition, this day is known as "Clean Monday" — and it comes on the sixth Monday before Palm Sunday. 

The name Clean Monday stems from the beginning of a season of fasting and a renewed sense of forgiveness, Archpriest John Whiteford of St. Jonah Orthodox Church in Spring, Texas, told Fox News Digital. 

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"In a certain sense, Orthodox Lent begins on Sunday, because what we do is after the liturgy, most parishes do what's called Forgiveness Vespers," Whiteford said in a phone interview. 

This is the first Lenten service, said Whiteford. The colors of the church are even switched from gold to black, and the vespers are sung using the Lenten melody. 

"In most parishes, at the end of that service, they will do what's known as the ‘Rite of Forgiveness,’" he said, which involves a request for forgiveness by all those present of everyone else. 

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"By the time the service is over, everybody has asked for everybody else's forgiveness," said Whiteford. 

On Clean Monday, most parishes have a service in the evening called the "Great Compline and the Great Canon of St. Andrew of Crete," he said, "which is a penitential canon." 

The canon is prayed over four parts during the first week of Lent.

"The Great Compline is a fairly long service, when it's done as part of Lent," he said. "It has a lot of penitential elements to it. But the canon is the highlight of that service." 

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While Orthodox and other Eastern Christians fast during Lent, these fasts are more intense during the first week, Whiteford told Fox News Digital. 

Orthodox Christians fast from meat and dairy products throughout Lent, including fish. 

Whiteford said fish is permitted only on Palm Sunday and on the Annunciation. (The consumption of shellfish and insects is, however, permitted during this time.)

During the first week of Lent, many people take the fast a step further and avoid things such as foods cooked in oil, he said.

"They're trying to eat very simply," said Whiteford.

He said some people choose to eat soup and bread during this time. 

"And some people won't even eat at all, at least for the first three days." 

The average layperson, he said, will not keep a fast as intense as that after the first week of Lent — but fasting will ramp up once again come Holy Week, the week preceding Easter.

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Orthodox Easter this year will be celebrated on May 5, 2024, more than a month after "Western" Easter on March 31. 

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