Albert Barnes, Notes on Amos 5:18
“The Day of the Lord” had already become the name for every day of judgment, leading on to the Last Day. The principle of all God’s judgments is one and the same. One and the same are the characters of those who are to be judged. In one and the same way, is each judgment looked forward to, neglected, prepared for, believing, disbelieved. In one and the same way, our Lord has taught us, will the Great Day come, as the judgments of the flood or upon Sodom, and will people prepared or unprepared, as they were then. Words then, which describe the character of any day of Judgment, do, according to the Mind of God the Holy Spirit, describe all, and the last also.