As the Christian faith spread throughout the world, local groups of Christians would celebrate the liturgical year in a slightly different way. Such is the case with Orthodox Christians (as well as many Byzantine Catholics) and Roman Catholics in their observance of Lent.
Here are five basic differences between Orthodox and Catholic Lent. As a note, these are "general" rules, as there are many different Orthodox and Byzantine churches and each have their own local traditions.
No Ash Wednesday
Roman Catholics begin Lent with the celebration of Ash Wednesday and the imposition of ashes upon the forehead.
Eastern Christians generally begin their observance of "Great Lent" with "Forgiveness Sunday."
The Catholic Telegraph gives a brief summary of what happens during the liturgical ceremony, “During Forgiveness Vespers, the clergy and the congregation ask forgiveness of each other, one person at a time, for offenses they committed, knowingly or not, and exchange a kiss of peace.”
It is also called, "Cheesefare Sunday," as it is the last day before the Lenten fast, which includes fasting from dairy products.
No daily Mass
While Roman Catholics continue to celebrate the Holy Eucharist on every day of the week, most Eastern Christians do not celebrate the Divine Liturgy during the week.
In place of the Divine Liturgy, on Wednesdays and Fridays the Liturgy of the Pre-Sanctified Gifts is celebrated. It is essentially the Liturgy of the Hours with a communion service.
Roman Catholics have a similar communion service on Good Friday.
Great Fast
Eastern Christians have much stricter rules for fasting during Lent, fasting from meat, eggs, and dairy, and not drinking any alcohol.
This is much more than Roman Catholics, who are only required to fast (limit the size of meatless meals) on Ash Wednesday and Good Friday, and abstain from meat on Fridays during Lent.
Alleluia is used more often during Lent
Roman Catholics are accustomed to "burying" the Alleluia during Lent, not reciting it until the Easter Vigil.
For many Orthodox and Byzantine Christians, Alleluia is used more frequently during Lent, as it is seen as a word of praise that is rarely omitted.
Purple only used on weekdays
The color purple (or violet), is most often used in Orthodox churches on the weekdays of Lent, while bright colors (usually gold) are used on Sundays during Lent.
Bright and joyful colors always mark celebrations of the Divine Liturgy, which is why Sundays retain that color even during Lent.
For Roman Catholics, violet is used every day of Lent, except on major feasts and on the 4th Sunday of Lent, when rose is used.