Leviticus 16:1

‘And Yahweh spoke to Moses, after the death of the two sons of Aaron, when they drew near before Yahweh, and died,'

These words of Moses are timed as taking place after the death of the two sons of Aaron in Leviticus 10:1. They had drawn near before Yahweh and died because they offered what was false and behaved foolishly. Now it was necessary that the High Priest offered what was true, otherwise he too would die. But the laws of uncleanness had previously been expounded on in order to fill out the need for this day by stressing the daily uncleannesses of Israel. It explained how a holy God could continue to ‘dwell' in a camp of such uncleannesses. For in spite of the extreme efforts made to preserve the holiness of the Sanctuary, it could not avoid being to some extent tainted by surrounding and sometimes hidden and/or unconscious uncleanness.