No priest, neither ordinary priest nor the High Priest himself, was fit to serve in the Temple unless he was wearing the priestly garments. The lay, or ordinary, priests must wear four garments - the tunic, the belt, the turban and the pants. These garments were made from white flax; hence they were referred to as the "garments of white," and they were also worn by the High Priest on the Day of Atonement.   More about the priestly garments...

Also see The Service of the Priests and the Levites.