Midrash su Levitico 8:34

37) (Vayikra 8:34) "As He has done this day, so has the L–rd commanded to do to make atonement for you": From here they said: Seven days before Yom Kippur (corresponding to the seven days of the miluim) the high-priest is separated from his house to the "counselors' cell" (in the Temple). And just as they separated the high-priest from his house, so they separated the Cohein who was to burn the red heifer from his house. And why would they separate him for seven days? Lest his wife be found to have possibly been a niddah (during intercourse) and he be disqualified from (the atonement service) all seven days. (Vayikra 8:34) "the L–rd has commanded to do to make atonement for you": He said to them: This service shall atone for you until the resurrection.