As the Old Testament closes with Malachi 4, there is the promise of the return of Elijah.
While it will be a glorious day, it will not be glorious for everyone. There will be great separation. Evil doers will be separated from those who revere the name of God (meaning that revering the name of God requires doing good).
A thousand years had passed between Moses and Malachi, but the law of the Lord had not changed. Though through Jesus God will make a new covenant with His people, the basic law of the Lord would not change. In fact, Jesus said the law would not pass away until the end of the earth.
But before Jesus would come, a herald would appear in the spirit and power of Elijah. We know him as John the Baptizer (Luke 1:17) and his job would be to ready people for the coming day of the Lord.
The “day of the Lord” is a day of division, sheep divided from the goats, evil doers from the faithful. For some it’s a horrible day. For others, the prepared, it is a blessing. The old hymn asks: “Are you ready for that day to come?”