Who shall also confirm you unto the end, that ye may be blameless in the day of our Lord Jesus Christ.
Who - God (1 Corinthians 1:4), not Jesus Christ (1 Corinthians 1:7), in which case it would be, 'in His day.'
Unto the end - namely, 'the coming of Christ.'
Blameless in the day of ... Christ (1 Thessalonians 5:23). After that day there is no danger (Ephesians 4:30.) Now is our day to work, and the day of our enemies to try us; then will be the day of Christ, and of His glory in the saints.