Not all of the Reformed persuasion are Amillennial (a misnomer of the position in my humble opinion). One must take the epistles to the Revelation, not the other way around. As far as the literal (by your meaning of the word, 'literal') event prophesied in the NT that must occur before the Day of the Lord, is the falling away, and the revelation of the son of perdition, 2 Thessalonians 2:3. The apostacy will be on a scale never before seen. Almost total. Jesus Himself wondered if He would find faith in the earth when He returns, Luke 18:8
This corresponds with the end of the Thousand Years in Revelation and the loosing of Satan. And since Revelation is best understood when not read as a linear 'history', so to speak, it also corresponds to Revelation 13. There is not one beast, but two working in tandem, and so the 'man of sin' is best understood as a system, or a power, but unlike anything seen before the binding of Satan, and they will have power of Satan to make war with the saints. The devils won't be subject like they were in Luke 10:17. (Notice Christ's response and how it sounds like Revelation 12:3).
The Reformers, with good reason, began to conclude, that Rome was the whore, was Babylon, and was the Antichrist.
But Peter said that even in his day there were antichrists in the earth. These events symbolized in Revelation have played out in microcosm all over the earth age after age.
We're given another picture of the end of the world, and that is the Ark. As it was in the days of Noah, life went on pretty sweetly. They did eat, they drank, they married wives, they were given in marriage,and they despised and rejected Noe and his family, until the day that Noe entered into the ark, and the flood came, and destroyed them all.
In microcosm, in the days of Lot, the same day that Lot went out of Sodom it rained fire and brimstone from heaven, and destroyed them all.
And so, God has His elect from every nation and every age of the earth, and when the last of them enter the Ark, that is, enters into union with Christ, then the Way will be shut, and judgement will rain from Heaven.