Blessed is He that cometh in the name of the Lord. Hosanna in the highest!
Palm Sunday of the Lord's Passion (the full name), the first Sunday of Holy Week within the Lenten Season, commemorates Jesus' triumphal entry into Jerusalem preceding his passion. As he entered, the people of Jerusalem recognized Jesus as their king, saying "Hosanna to the Son of David; Blessed is he who comes in the name of the Lord!"
Traditionally in the Western Church the Palm Sunday service begins with the "blessing of the palms," where the palms used in the procession that follows are blessed. It is during this time that the story of Jesus' triumphal entry into Jerusalem is read. Then a procession into the church building follows. If there cannot be a procession from the outside of the church, a solemn entrance, taking place entirely within the church, may be done. The hymns and psalmody are related to Christ's office as King. Traditionally the Gloria Laus (i.e. All Glory Laud and Honor), written by Theodulf of Orleans, is sung. Many times the worship service contains a "preaching of the passion," where different events in the last days of Christ are read publicly within the Eucharistic service. In modern Catholic services, the priest and/or a combination of readers read aloud Matthew 26:14-27:66 (Year A), Mark 14:1—15:47 (Year B), or Luke 22:14-23:56 (Year C).
Various customs have developed to celebrate Palm Sunday. In the Slavic countries, the faithful walked through their buildings and fields with the blessed palms, praying and singing ancient hymns. They then laid palm pieces on each plot of ground, in every barn, building, and stable, as a petition was made for protection from weather and disease, and for a blessing upon the produce and property.
The pilgrim Egeria attests to a Palm Sunday procession taking place in the Jerusalem Church at the end of the 4th century. In the Gallican Bobbio Missal of the 8th century we find a reference to blessing of the palms, which symbolize the victory of Christ. The more elaborate celebrations of the Middle Ages have been replaced by simpler services in the Western Church. Many denominations, including Lutherans, Methodists, and Presbyterians celebrate Palm Sunday, in addition to Catholics and Eastern Christians. In most churches, the ashes for Ash Wednesday are derived from burned palms, left over from Palm Sunday liturgies.