There are a variety of events that are clustered on this last day before Jesus was arrested that are commemorated in various ways in services of worship.
These include the last meal together, which was probably a Passover meal, the institution of Eucharist or Communion, the betrayal by Judas (because of the exchange with Jesus at the meal), and Jesus praying in Gethsemane while the disciples fell asleep.
It is observed in many Christian churches as a time to commemorate and enact the suffering (Passion) and death of Jesus through various observances and services of worship.
It is only in walking through the shadows and darkness of Holy Week and Good Friday, only in realizing the horror and magnitude of sin and its consequences in the world incarnated in the dying Jesus on the cross, only in contemplating the ending and despair that the disciples felt on Holy Saturday, that we can truly understand the light and hope of Sunday morning!
In observing this truth, that new beginnings come from endings, many people are able to draw a parable of their own lives and faith journey from the observances of Holy Week.
Red is the color of the church, used for Pentecost as well as remembering the martyrs of the church.
Since it symbolizes shed blood, it is also used on Palm Sunday to symbolize the death of Jesus.