On the morning of Holy Thursday, the Cathedral becomes the authentic liturgical centre of the entire Ambrosian Church: here the entire diocesan presbytery converges to celebrate, together with the Archbishop, the solemn Chrism Mass, during which Chrism is consecrated and the Oil of Catechumens and the Oil of the Sick are blessed.
White is the colour of the vestments used by the Archbishop, the only ritual exception during these days. AnAmbrosian characteristic throughout Holy Week is, in fact, the use of the red liturgical colour: rather than the penitential aspect, which dominated during Lent, the accent shifts to the Mystery of the Body and Blood of Christ, as well as to the centrality of the Cross and of the redemptive Passion.
At the offertory, together with the bread and wine, the amphorae containing the Oils are carried in procession to the altar, whose blessing has preserved, in the Ambrosian liturgy, its traditional location: before the Eucharistic Prayer, the consecration of the Chrism and the blessing of the Oil of Catechumens, and only before the Eucharistic Prayer concludes the blessing of the Oil of the Sick.
At the moment of consecrating the Chrism, the Archbishop infuses the amphora with balms and perfumed essences, and carries out the ancient rite of insufflatio, blowing on the oil to be consecrated.
At the end of the Mass, the blessed Oils are taken to the Sacristy of the Cathedral, where they remain available to all the Parishes of the Diocese, for administration of the Sacraments throughout the year.
The evening celebration "in the Lord's Supper" begins the Easter Triduum, in which the Passion, Death and Resurrection of Jesus are commemorated.
The Mass is preceded by the rite of the washing of the feet which, recalling the humble gesture with which Jesus prefigured the total gift of himself in his Passion, proclaims the primacy of service.
The liturgy traces the initial moments of the Lord's Passion, from the Last Supper to Peter's denial. The Eucharistic celebration is part of the Vespers prayer and opens with the rite of the skylight, accompanied by the lighting of the church and the offering of incense. After the proclamation of the Passion according to Matthew and the homily of the Archbishop, the antiphon Cenætuæ is sung: a very ancient text, directly translated from a Byzantine original of the sixth century, which only the Ambrosian tradition has in the West. It recalls the mystical Supper to which Christ invites the faithful and the treacherous kiss of Judas. The choreographic frame in which this song takes place is also evocative, introducing us to the Eucharistic liturgy: in fact, it is performed by the puericantores of the Musical Chapel, arranged in a crown around the altar, symbol of Christ. Similar to what happens in the Easter Vigil, the Milanese liturgy has also retained a text just for the Eucharistic Canon.
Finally, after Communion, the Archbishop returns the Eucharist in a procession to its new position, at the side altar of Our Lady of the Tree, where the faithful will file pass in silent adoration until the beginning of the Easter Vigil. In ancient times this rite was celebrated "pro sepultura dominica rapræsentanda", almost as if to re-present the Burial of the Lord: from here the devotion to the visit to the "Sepulchres" was generated, especially in the sphere of popular religiosity. A small detail survives of this allegorical reading of the repositioning of the Eucharist: during the procession, the Archbishop covers the pyx wrapping it with the edges of the humeral veil, alluding to the gesture made by Joseph of Arimathea who, according to the Gospels, wrapped the body of Jesus in a shroud, before placing it in the tomb.