Leviticus 16:4

He shall put on the holy linen coat - He was not to dress in his pontifical garments, but in the simple sacerdotal vestments, or those of the Levites, because it was a day of humiliation; and as he was to offer sacrifices for his own sins, it was necessary that he should appear in habits suited to the occasion. Hence he has neither the robe, the ephod, the breastplate, the mitre, etc.; these constituted his dress of dignity as the high priest of God, ministering for others and the representative of Christ: but now he appears, before God as a sinner, offering an atonement for his transgressions, and his garments are those of humiliation.

The high priest when he changed his dress on this day was required to bathe himself. In his "golden garments" he had, on this day, and for the previous week, to offer the regular daily sacrifices, and to perform the other sacerdotal duties of the sanctuary, which were usually performed by a common priest. The dress of white linen, which he now put on, appears to have been like the ordinary dress of the common priests, except in the substitution of a linen mitre for the bonnet (or cap), and of a plain linen girdle for the variegated one (Exodus 28:40-43 notes). In preparing to enter the holy of holies, he attired himself in spotless white as a token of the holiness without which none, in a spiritual sense, can enter the divine presence. He thus became a more distinct foreshadow of the greater high priest Hebrews 7:26; Hebrews 6:19-20. This significance belonged to the high priest only in his official capacity as mediator: in his own person he had infirmity, and was required "to offer up sacrifice, "first" for his own sins, and then for the people's." Hebrews 7:27. See the notes at Leviticus 9:7-14. On the same ground it was that, although as a mediator he had to enter the most holy place, as sinful man he needed the cloud of incense as a veil to come between him and the holiness of Yahweh. See Leviticus 16:13.

16:4 The linen coat - It is observable, the high - priest did not now use his peculiar and glorious robes, but only his linen garments, which were common to him with the ordinary priests. The reason whereof was, because this was not a day of feasting and rejoicing, but of mourning and humiliation, at which times people were to lay aside their ornaments.These are holy - Because appropriated to an holy and religious use.