“He shall put on the holy linen coat, and he shall have the linen breeches on his flesh, and shall be girded with the linen girdle, and with the linen mitre shall he be attired. They are the holy garments. And he shall bathe his flesh in water, and put them on.”
But before presenting these the High Priest had to divest himself of his normal Priestly garments and, after thoroughly washing himself, put on the special garments only used on the Day of Atonement. These were pure white, and consisted of the holy linen coat, the linen breeches covering his ‘flesh' (his unseemly parts), the linen girdle, and the linen headdress. These were the holy garments. And before donning them he had to wash himself thoroughly with water, this is spite of the fact that he had already offered the morning sacrifice and had probably not left the tabernacle since. All traces of earthiness had to be removed. He was about to enter the Holy of Holies.
The reason for having to wear these special garments was probably: 1). Because they had to be pristine in order for him to enter the Holy of Holies. His ‘every-day' High Priestly clothes, in all their splendour, were not sufficient. They were tainted. 2) Because he could not enter God's presence on that day in garments ‘for glory and for beauty' because he was coming as a penitent sinner and a suppliant. 3). Because this was a day on which he and Israel would be made ‘white'. 4). To emphasise the holiness of life required of the High Priest.