LEV110 After having been spiritually impure, a Kohen who has immersed in a mikvah during the day shall not serve in the Beis HaMikdash until after nightfall. A Kohen is an emissary between his fellow Jews and Hashem. When he brings offerings in the Sanctuary his brethren regain closeness to Hashem for they gain atonement for their sins. Therefore, while performing his duties in the Sanctuary the Kohen must be spiritually pure. If he loses his spiritual purity and begins the process of restoring it, only Hashem knows when the Kohen completely regains his purity and is again fit to serve in the Sanctuary. Hashem reveals to us that the time comes with the appearance of three stars on the night after the Kohen’s ritual immersion.