This Friday, April 18, is Good Friday, the most somber day in the Christian calendar.

The date of Good Friday changes every year, but always occurs between March 20 and April 23, and always falls on the Friday before Easter Sunday, according to ShareFaith.com. 

Good Friday marks the peak of Holy Week, and is held in solemn reverence.

Good Friday is the day Christians believe Jesus was crucified on the cross. Christians believe that Jesus died for their sins, and therefore mourn the sin of man, yet rejoice upon God sacrificing his son to redeem humanity's sins.

The church initially set aside every Friday as a day of prayer and fasting as early as the first century. But it was not until the fourth century that the church began observing the Friday before Easter as the day that is associated with the crucifixion of Jesus.

The holiday was first called "Holy" or "Great Friday" by the Greek Church, then the name "Good Friday" was adopted by the Roman Church in the sixth or seventh century.

The name "Good Friday" is said to have possibly derived from the Gallican Church in Gaul (modern-day Germany and France). The name "Gute Freitag" means "Good" or "Holy" Friday in German. Some also believe that it derives from the name "God's Friday," and "good" was used to replace "God," in order to not use their lord's name in vain.

Good Friday is observed in a solemn fashion, with priests and deacons wearing black vestments. The pulpit and the altar are bare, and no candles are lit. Many churches also hold special services to commemorate the day's significance.

Anytime between midnight and 3 a.m. on Good Friday, priests and clerics recite specific prayers. During morning ceremonies, priests and church officials recite lessons from scriptures. Afterwards, they pray for God to forgive mankind.

At noon there is the Adoration of the Cross, which is when a representation of the True Cross is unveiled, and the clergy and congregants pay homage to Christ's sacrifice.

Then there is the Mass of the Presanctified, in which the priest takes Communion from the host that was blessed during the Maundy Thursday ceremony. The ceremony concludes at 3 p.m. with a procession, followed by evening prayers.

In the Protestant faith, Good Friday begins at noon and lasts until 3 p.m., which are the hours during which it is believed that Jesus hung on the cross. The services often end with the speaking of the last phrases that Jesus spoke.

Some churches also reenact Jesus's crucifixion according to the Gospel of John, as well as the processions of the Stations of the Cross.