Some People were resurrected to physical life at the time of Jesus's death but not to eternal life just yet.
The Day of the Lord generally refers to the end time events surrounding the return of Jesus Christ to establish God's Kingdom on earth.
The term indicates the end of man's rule and the beginning of the age of Jesus Christ's rule.
This is the time the saints will be resurrected to eternal life to help rule the Kingdom with Jesus.
And the rest of the dead in all of history will be given an opportunity for eternal life as well.