The Tabernacle needed to be cleansed once a year on the Day of Atonement because it was defiled by the sins of the Israelites [Lev 16:16].

1. The Tabernacle was defiled by the uncleanness of the Israelites in whose midst it was [Lev 15:31; 16:16].
2. The defilement of a member of the community defiled the Tabernacle [Num 19:13; 35:34]
3. The Altar was cleansed from the sins of the Israelites with blood on the Day of Atonement [Lev 16:19]

How Was the Tabernacle of Yahweh Defiled by the Sins of the Israelites?

1. Just as the land is defiled by blood [Num 35:33; Psa 106:38], so too, it is defiled by sin.
   1. The land of Canaan was, for example, defiled by forbidden sexual relations of its inhabitants [Lev 18:24-28]
      1. Sexual relations dishonoring close relatives, with animals, or homosexual relations [Lev 18:6-24]
      2. A man taking his divorced wife back after she had married another man [Deut 24:4; Jer 3:1]
   2. A man who hangs on a tree is cursed and if he remains on the tree overnight, the land itself will be defiled [Deut 21:23]
   3. Idolatry defiles the land [Jer 3:9]
2. A defiled house or tent makes those in it unclean [Lev 14:44-47]

Therefore, since the land was defiled by the sins of the people living in it, the Tabernacle being in the same land among the Israelites was defiled by their sins.