The evening of the Last Supper begins the Triduum. Christ washes his disciples' feet, institutes the Eucharist, and enters the agony of Gethsemane.

The liturgical customs of Maundy Thursday are among the richest in the Christian year. The stripping of the altar after the evening Eucharist — removing cloths, candles, and ornaments to leave the sanctuary bare — dramatically enacts the desolation of Gethsemane and the abandonment of Christ by his disciples. This practice is attested from at least the seventh century (Ordo Romanus) and may be older. The washing of feet, whether performed by the presider on members of the congregation or as a mutual act, recovers the dominical command of John 13 and has been practiced in both East and West, though with varying emphasis — in England, the monarch's participation in the Royal Maundy is documented from the thirteenth century. The reservation of the Blessed Sacrament and the watch before the altar of repose recall Christ's words in Gethsemane: 'Could you not watch with me one hour?' (Matthew 26:40). The Tenebrae office — a service of progressive darkness in which candles are extinguished one by one — is historically associated with the Triduum evenings, and the BCP 2019 provides for it on this night. The evening marks the beginning of the Paschal Triduum — the three-day liturgy spanning Thursday evening through Easter Day that forms the single most important liturgical event of the Christian year. The collect focuses on the institution of the Eucharist specifically, grounding the day's devotion in sacramental theology: what was given on this night continues to be received at every celebration of the Lord's Supper.

Maundy Thursday commemorates the events of the night before Christ's crucifixion: the Last Supper, the institution of the Eucharist, the washing of the disciples' feet, and the agony in Gethsemane. The name 'Maundy' derives from the Latin mandatum novum — 'a new commandment' — from John 13:34, spoken after Jesus washed his disciples' feet. The earliest evidence of a distinct liturgical observance on this day comes from the late fourth century. The pilgrim Egeria, writing around 384, describes elaborate Jerusalem services including an afternoon Eucharist at the Martyrium basilica and an evening station at the Imbomon (the traditional site of Jesus's teaching on the Mount of Olives), concluding with a nighttime procession to Gethsemane. By the time of the Apostolic Constitutions (c. 380), the day was established as a fast day. Augustine of Hippo (Ep. 54, to Januarius) attests that by around 400 the day was celebrated with both a morning and evening Eucharist — the evening celebration deliberately recalling the hour of the Last Supper. The foot-washing ceremony (mandatum or pedilavium) became a standard Western liturgical practice by the early medieval period. The seventh-century Ordo Romanus documents the stripping of altars, and the Gelasian Sacramentary provides proper prayers for the day. The Eastern churches developed the day under the name 'Holy and Great Thursday,' with emphasis on both the Last Supper and the Passion beginning at Gethsemane.