Veiling of the Altar, the Chalice and the Tabernacle
In the Old Testament the Holy of Holies was veiled. This is where our tradition of veiling Holy things began.
The former [Old Covenant] indeed had also justifications of divine service and a sanctuary. For there was a tabernacle made the first, wherein were the candlesticks and the table and the setting forth of loaves, which is called the Holy. And after the second veil, the tabernacle which is called the Holy of Holies: Having a golden censer and the ark of the testament covered about on every part with gold, in which was a golden pot that had manna and the rod of Aaron that had blossomed and the tables of the testament. And over it were the cherubims of glory overshadowing the propitiatory: of which it is not needful to speak now particularly. Now these things being thus ordered, into the first tabernacle, the priests indeed always entered, accomplishing the offices of sacrifices. But into the second, the high priest alone, once a year: not without blood, which he offereth for his own and the people’s ignorance: The Holy Ghost signifying this: That the way into the Holies was not yet made manifest, whilst the former tabernacle was yet standing.
…The Ark of the Old Covenant was kept in the veiled Holy of Holies. And at Mass, what is kept veiled until the Offertory? The Chalice — the vessel that holds the Precious Blood! The most recent General instruction of the Roman Missal states that “it is a praiseworthy practice for the chalice to be covered with a veil. (GIRM #118.)
And, between Masses, what is veiled? The Tabernacle, the vessel which holds the very Body of Christ. These vessels of life are veiled because they are sacred and holy! The Church stipulates that the primary way of indicating the presence of the Blessed Sacrament in the Tabernacle is by veiling it. The presence of the Eucharist in the tabernacle is to be shown by a veil or in another suitable way determined by the competent authority. (Roman Ritual: Holy Communion and Worship of the Eucharist outside of Mass, 11).
The Altar likewise must always be covered (GIRM#297), with the exception of when it is stripped after the Mass of Holy Thursday.
We should not be surprised, therefore, to find that the importance of the sacred was reaffirmed in the “Third Instruction on the Correct Implementation of the Constitution on the Sacred Liturgy” (1970) which states: “Liturgical reform is not synonymous with so-called desacralization and should not be the occasion for what is called the secularization of the world. Thus the liturgical rites must retain a dignified and sacred character.” The difficulty is that when the sacred mysteries are unveiled, they lose their sacred character. Tradition teaches us that “a mystery—the tremendum—in order to be a mystery, needs to be hidden, so that we may long for it to be revealed.”