Traditionally on Holy Thursday in St. Peter's Basilica, there was a curious ceremony which took place, the ablution (or washing) of the papal altar with water and wine. Cardinal Wiseman, in his lectures on the offices and ceremonies of Holy Week in the papal chapels, describes the ceremony accordingly:
...another point of ancient practice, once probably common to every church, but now hardly observed except in St. Peter's. The altars are everywhere formally stripped on Holy Thursday and remain uncovered until the following Saturday.C.M. Baggs in his work, The Ceremonies of Holy Week at the Vatican and S. John Lateran's Described, describes the ceremony further:
During Tenebrae on Thursday evening, each of the canons and other functionaries of St. Peter's, receives a brush curiously made of chip, and after the office, the entire chapter proceeds to the high altar, where seven flagons of wine and water have been prepared. These are poured upon the altar, and the canons, passing six at a time before it, rub it all over with their brushes, after which it is washed with sponges and dried.
Saint Isidore of Seville, in the seventh century, mentions the custom of washing the altars, and even the pavement of the church on this day, in commemoration of that act of humility, by which our Redeemer washed his disciples'feet and St. Eligius records, in similar terms, both the practice and the motive. The Roman Ordo, Abbot Rupert, and other writers, speak of this ceremony as commonly practised and many documents of the middle ages show it to have been observed at Siena, Benevento, Bologna, and other churches. It was no less practised in England; for the Sarum Missal thus describes it: "After dinner, let all the clerks meet in the church to wash the altars. First, let water blessed out of choir and privately. Then let two of the most dignified priests be prepared, with a deacon and subdcacon and two acolytes, all vested in albs and amices, and let two clerks bear wine and water, and let them begin with the high altar and wash it, pouring thereon wine and water. "After a minute description of the prayers to be said in the course of the ceremony, the rubric proceeds:"After the gospel has been sung as at mass, the two aforesaid priests shall wash the feet of all in choir, one on one side and another on the other, and then shall do the same mutually.
These examples will suffice to show how the ceremonies of Holy Week, as performed in the Vatican, have preserved rites formerly very general in the Church, but which would have been almost entirely lost in practice, had they not been here jealously observed.
There slill remains another remarkable ceremony customary in S. Peter's on Holy Thursday. After the office of Tenebrae, the chapter of that basilica proceeds in procession from the chapel of the choir to the high altar. The black stoles which six of the canons wear, and the yellow [unbleached] and extinguished tapers of the acolytes, are signs of mourning for the sufferings of Christ. They all carry elegant aspergilll of box or other wood, and having prayed for a short time in silence, they chant the anthem, "They divided my garments etc." and the psalm "0 God, my God, why hast thou abandoned me?" A fine cloth, which covered the altar, is then removed from it, and the Cardinal-priest of the church and the six canons pour wine upon the altar, and wash it with their aspergilll or brushes. After the other canons, beneficed clergymen, etc. have in turn washed it in like manner, the Cardinal and the six canons begin to dry it with sponges and towels; all then kneel down, and the ceremony comcludes with the verse "Christ became obedient unto death etc." the Our Father, and the prayer of the day "Look down, we beseech thee etc." The chapter then venerates the relics shown as usual from the gallery above S. Veronica's statue."Here are some (cleaned up) illustrations of this ceremony, coming from Rerum Liturgicarum.
For a bit more detail yet, we turn to The Ceremonies of Holy Week in the Papal Chapel at the Vatican, excerpted from the work of estimable Francesco Cancellieri:
In the Chapel of the Choir the same functions take place as in the pontifical chapel and with the same solemnity. The Lamentations and Miserere are sung by the choristers. But the most remarkable ceremony is the washing of the altar.
While Matins and Lauds are singing in the Choir, a table is prepared near the high altar on which are placed seven crystal vases, and one of gilt copper containing wine. In one are seven towels, in another seven sponges: when the choir sings the « Benedictus » of Lauds, small brushes of box or yew but generally of bloodwort, arranged in the form of a diadem, are distributed to the canons.
The sacristan of the choir prepares a cope and seven black stoles for the seven senior canons, who repair to the high altar, preceded by two Acolytes bearing the Cross which is veiled in black, with the candles extinguished, in sign of mourning. The Cross bearer and Acolytes are placed on the eastern side of the Altar, the whole chapter is disposed in a circle, when all kneel and pray. The antiphon Diviserunt sibi « they divided my garments » is followed by the psalm Deus, Deus meus etc.
The Altar is uncovered, and washed with wine and then with water by the six first Canons who are followed by the rest of the chapter. This ceremony being finished, the seven sponges are brought to wipe, and the seven towels to dry the Altar. The Clergy repeat the Antiphon « Diviserunt sibi,» the «Christus factus est» ; the « Pater noster » is said in a low tone, the prayer Respice quaesumus Domine is recited. All then kneeling venerate the three principal relics, the Cross, the Holy face, and the Lance, preserved in the gallery over the statue of St. Veronica.
