These are extraordinarly rare images which show the Mass of Holy Thursday with Pope Pius XI in the Sistine Chapel. (Something even more extraordinarily rare will be forthcoming in the next day or two. Watch for it.) Further, one sees the Maundy Thursday altar of repose in the Cappella Paolina.
The Mass is a Pontifical Mass coram Summo Pontifice. (Clicking the link will give a greater description of this, but a few brief ceremonial descriptions are provided below for those interested in understanding what they are seeing, and a further source is quoted following the pictures.)
(The Pope is at the throne in papal mantum with two Cardinal Deacons in cappa magna on either side.)
(At the altar, one sees the Cardinal celebrant -- who when seated, would sit upon faldstool on the Epistle side of the altar. One can also see the deacon, subdeacon and assistant priest as per solemn pontifical liturgies. One will note the white veil covering the cross on Holy Thursday.)
(The cardinals attend.)
(The altar of repose in the Pauline Chapel)
For those whose historical curiousity runs deeper, a further supplement may be of interest. In The Ceremonies of Holy Week in the Papal Chapel at the Vatican by Francesco Cancellieri, he notes the following of these ceremonies as they stood in the early 19th century surrounding Holy Thursday, which give some further context to the photographic images above:
This day Mass is celebrated by the senior Cardinal; the altar and cross are covered with white veils, the candles are white.
The Pope wears a white cope and mitre of cloth of gold and formal, with a figure of the Holy Ghost in the centre. At the beginning of Mass, the Cardinals pay their homages. The "Introit" and "Kyrie" are sung.
Before the elevation, twelve Esquires dressed in red and carrying lights kneel before the altar.
Two hosts are consecrated, one being kept for the following day, and two Masters of Ceremonies distribute candles to those who form the procession. The "Agnus Dei" is followed by the dona nobis pacem and the Celebrant having replaced the holy Sacrament, the Postcommunion and Deo Gratias are sung in plain chant...
Procession to the Pauline Chapel where the Sepulchre is Placed
Mass being finished, the Celebrant returns to the sacristy, and the Cardinals change their dress. The procession moves in the same order as on Palm Sunday to the Sala Regia.
On the entrance of His Holiness into the [Pauline] chapel, illuminated by 567 wax candles, the Verbum Caro is sung, and on arriving at the altar the Pope delivers the chalice containing the Sacrament to the Cardinal deacon who deposits it in the sepulchre where it is incensed by the Pope; Monsignor Sacrista locks the sepulchre and delivers the key to the Cardinal Penitentiary who is to officiate on the following day.
The procession passes through the door near the Pauline chapel to the gallery in front of church, to which the Pope is carried under a canopy supported by eight Referendaries.
The bell of St. Peter's, the music of the regiments stationed in the piazza, the firing from the Castle St. Angelo, simultaneously announce the triple blessing of the Holy Father...
Washing the feet of XIII pilgrims
The Cardinals having resumed their violet colored dress, precede his Holiness to St. Peter's which is decorated with damask... for the ceremony of washing the feet called Mandatum, our Saviour having given us the example and command.
The Pontiff, having resumed the violet stole, the red satin mantle and the formal with the silver mitre, repairs to the seat prepared for him, and gives his blessing to the Cardinal Deacon appointed to celebrate the service...
The Holy Father wearing a linen apron... then washes the feet of XIII priests or deacons called Apostles, who are seated on high benches.
A Subdeacon in a white tunic without a maniple, attends on the Pontiff's right, who kneeling, washes the right foot of each in a silver gilt basin presented by an Esquire, wipes the foot and kisses it. Two prelates support the mantle, two extra-Chamberlains follow with silver basins, one containing XIII towels, the other nosegays, each Apostle receiving one. The Treasurer holding a purse of crimson velvet fringed with gold, presents to each a gold and silver medal...
Table for the Apostles; The Pope serves
The Apostles are afterwards conducted into one of the halls of the Vatican where the table is prepared; prayers being said the Pope gives his blessing, an apron is tied round him, he pours water on the hands of the Apostles, presents them sundry dishes; offers them drink one or twice, gives another blessing and retires.