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Good Friday is the first day of the Easter Triduum and the day that Catholics and other Christians throughout the world commemorate the crucifixion of Jesus. According to Mark 15:42 Jesus died "on the day of preparation, the day before the sabbath." The Hebrew Sabbath is celebrated on Saturday which is preceded by Friday. Therefore the Friday before Easter (the day that we celebrate Jesus' resurrection from the dead) is traditionally marked as the day Jesus died on the cross.
Why don't we celebrate Mass on Good Friday?
The Good Friday liturgy has three parts: the Liturgy of the Word, the Veneration of the Cross, and the Service of Communion.
The Mass itself is the unbloody re-presentation of Jesus' death on the cross at Calvary. The priest, and victim of the sacrifice are the same person, Jesus, at both Calvary and every Mass. From the earliest days, Good Friday has been kept as an aliturgical day, that is one on which we do not celebrate Mass. The Eucharist that is received on Good Friday was consecrated a day earlier at the Holy Thursday liturgy.
During Good Friday services you might see Catholics go up to the front of the church and bow before or even kiss a cross. This act is called Veneration of the Cross and in which Catholics honor the great sacrifice Jesus made on the cross.
At the end of the Good Friday service the ministers and assembly depart in silence.
Good Friday is also a day of fasting and abstinence (from meat).

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