The Jews never lost the conviction that they were God’s chosen people. They interpreted that status to mean that they were chosen for special privilege among the nations. They were always a small nation. History had been for them one long disaster. It was clear to them that by human means they would never reach the status they deserved as the chosen people. So, bit by bit, they reached the conclusion that what man could not do God must do; and began to look forward to a day when God would intervene directly in history and exalt them to the honour they dreamed of The day of that intervention was The Day of the Lord.
They divided all time into two ages. There was The Present Age which was utterly evil and doomed to destruction; there was The Age to Come which would be the golden age of God. Between the two there was to be The Day of the Lord which was to be the terrible birth pangs of the new age. It would come suddenly like a thief in the night; it would be a day when the world would be shaken to its very foundations; it would be a day of judgment and of terror. All over the prophetic books of the Old Testament and in much of the New Testament, are descriptions of that Day. Typical passages are Isa. 2:12; Isa. 13:6 ff.; Amo. 5:18; Zep. 1:7; Joel 2:1-32 ; 1Thess. 5:2 ff.; 2Peter 3:10.
William Barclay’s Daily Study Bible, ACTS, Acts 2.