The rubrical prescriptions of the Roman Missal, Breviary and “Caeremonial Episcoporum” for this time (Passiontide) are: before Vespers of Saturday preceding Passion Sunday the crosses, statues, and pictures of Our Lord and of the saints on the altar and throughout the church, with the sole exception of the crosses and pictures of the Way of the Cross, are to be covered with violet veil, not translucent, nor in any way ornamented. The crosses remain covered until after the solemn unveiling of the principal crucifix on Good Friday. The statues and pictures retain their covering, no matter what feast may occur, until the Gloria in Excelsis of Holy Saturday…The crosses are veiled because Christ during this time no longer walked openly among the people, but hid himself. Hence in the papal chapel the veiling formerly took place at the words of the Gospel: “Jesus autem abscondebat se.” Another reason is added by Durandus, namely that Christ’s divinity was hidden when He arrived at the time of His suffering and death. The images of the saints also are covered because it would seem improper for the servants to appear when the Master Himself is hidden (Nilles, “Kal.”, II 188). NewAdvent.org