Leviticus 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.