The final chapter of Prophet Malachi sets the distinction between those who follow the LORD, and those who don't. The 'day' referred here is the 'day of the LORD' - a time of judgement for the wicked and that of blessing for the righteous.
Those who revere LORD will experience unimaginable joy and go out fully energized like those well-fed calves.
Malachi 4:2 But for you who revere My name, the sun of righteousness will rise with healing in its rays. And you will go out and frolic like well-fed calves.The key to escape judgement and enjoy blessings of LORD's Kingdom is to obey the law of Prophet Moses.
3 Then you will trample on the wicked; they will be ashes under the soles of your feet on the day when I act,” says the LORD Almighty.
Prophet Elijah will be sent before the day of the LORD to serve as a warning. He will reconcile the turn the hearts of the fathers with that of the children.
Those who refuse to repent and reconcile will be dealt with the LORD. He will come and unleash His total destruction with frightening severity.
The final words of the LORD closes with a blessed promise, to all those who fear and honor Him.
A frightening warning, for those who decide not to listen to Him, concludes the Old Testament.