If, as I believe the scriptures teach, Christ died to atone for our sins, why did He die at Passover instead of the Day of Atonement?
The Day of Atonement is the one day of the year when the priest could enter the Most Holy Place (where the glory of God appeared over the Ark of the Covenant) and make sacrifices for his sins and sins of the people of Israel.
Passover commemorates the tenth plague of the Exodus and God's protection of Israel from it and their subsequent rescue from slavery in Egypt.
The former is natural day for Christ's sacrifice, right?
But that's not how it happened. I think the reason can be illuminated by considering what took place in that first Passover.
God did at least three things on that day:
1) He called a people out for Himself.
2) He freed His people from slavery.
3) He rescued His people from the wrath that He was going to show His enemies — in this case, Egypt.
In Christ's death, God did all of those things again.
Christ's blood "purchased men for God from every tribe and language and people and nation" (Rev 5:9) "by abolishing in his flesh the law with its commandments and regulations. His purpose was to create in himself one new man out of the two, thus making peace" (Eph 2:15). Now we are "a chosen people, a royal priesthood, a holy nation, a people belonging to God, that you may declare the praises of him who called you out of darkness into his wonderful light" (1Pet 2:9).
"We were in slavery under the basic principles of the world. But when the time had fully come, God sent his Son, ... to redeem those under law, that we might receive the full rights of sons. ... So you are no longer a slave, but a son; and since you are a son, God has made you also an heir" (Gal 4:3-7).
"The wrath of God is being revealed from heaven against all the godlessness and wickedness of men" (Rom 1:18) but "God did not appoint us to suffer wrath but to receive salvation through our Lord Jesus Christ" (1Thes 5:9).
The forgiveness of our sins, achieved by the shedding of that precious blood, is the vehicle by which the purposes of God are achieved. And so it was totally appropriate that Christ should atone for our sins on the day God rescued His people.