The concept of the "day of the Lord," particularly in the Old Testament, is that it is the day when the Lord will fulfill his purposes. In it, there will be a final judgment where God will be vindicated, and after the judgment there will be blessings. For instance, we read in Isaiah 13:9: Behold, the day of the Lord comes, cruel, with wrath and fierce anger, to make the land a desolation and to destroy its sinners from it (Isaiah 13:9). Joel says: "Alas for the day!" He means the day when the people of the earth will feel great regret because the Lord was very patient with them for centuries, and the day has come for God to reveal his righteousness. But, the day of the Lord also includes a time of blessing when God will fulfill his purposes of goodness and blessings for the people of the earth. And some terms, like "the new heavens and the new earth," are also part of the day of the Lord. So, there will be judgment for unbelievers, for evil, and for the enemies of people, and also blessings for the believers when God fulfills his promises to them. The theme of the "day of the Lord," is also present in the New Testament. For instance, in 2 Peter 3:10, it says: But the day of the Lord will come like a thief, and then the heavens will pass away with a roar, and the heavenly bodies will be burned up and dissolved, and the earth and the works that are done on it will be exposed (2 Peter 3:10). We see in Revelation, John says, "I was in the Spirit on the Lord's day." Some might think that "the Lord's day" is Sunday, but, in context, it's clear that here "the Lord's day" carries the same ideas as in 2 Peter because John is about to explain the difficult days that are coming. In this sense, the day of the Lord is a vision of what is going to happen in the future. This is why Paul says: "We rejoice in hope of the glory of God." One day God will reveal his purposes, and his nature, his character, his righteousness and perfect attributes. And this will be the day of the Lord.