The day of the LORD becomes the day of Christ
The Old Testament
The sun will be turned to darkness and the moon to blood before the coming of the great and awesome Day of the LORD.
Joel 2:31
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In the New Testament
For you are fully aware that the Day of the Lord will come like a thief in the night.
1 Thessalonians 5:2
BSB
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In the prophets, 'the day of the LORD' is the day on which God himself intervenes: great and terrible, and a day of darkness for those who felt secure. Isaiah: hide among the rocks, away from the terror of the LORD and the splendour of his majesty, when he rises to shake the earth.
Paul speaks in turn of 'the day of the Lord' and 'the day of Christ' or 'the day of our Lord Jesus Christ'. In the second letter to the Thessalonians he takes over Isaiah's words, 'from the presence of the Lord, and from the glory of his power', for the return of Jesus. So the old expectation of God's own coming moves to the coming of Christ. That fits Isaiah 40 and Malachi, where the messenger prepares the way for the LORD himself (no. 19).