The Book of Malachi is about the prophecies that Malachi spoke the Jews who had returned from Babylon to Judah. At this time, the Jews were discouraged even though they had completed building a new temple. The wonderful thing that previous prophets had promised for Judah had not yet happened. And the Persian Empire still ruled over them. As a result, they no longer were concerned about the law or worshiping God. Malachi rebuked them for not trusting in Yahweh. But he also promised them that Yahweh would do everything that he promised to do.
The prophet named Malachi wrote these messages from Yahweh to the Jews. However, since "Malachi" means "my messenger," it is possible that this was a title for this prophet. If so, his real name is not known to us.
Malachi spoke about "the day" several times. This is the same as "the day of Yahweh." In the Book of Malachi this expression referred when Yahweh would Yahweh would judge his people. He would remove those who are wicked and be a blessing to those who trusted in him. (See: [[rc://en/tw/dict/bible/kt/dayofthelord]] and [[rc://en/tw/dict/bible/kt/sin]])