Why is the Day of Atonement a mistranslation?
The term is called a mistranslation because it was not a day of atonement for Israel; there was no atonement until Christ died. It should have been called the Day of Covering, since the ritual pictured a covering rather than the actual atonement accomplished by Christ.
Les explains that the translators' choice of "Day of Atonement" is unfortunate, because it was not a Day of Atonement for Israel. No atonement existed until Christ died, so it should have been called the Day of Covering. This connects with the tabernacle types being fulfilled by Christ, our High Priest, who presented His blood and became our propitiation.