[except there come a falling away first]
(For me, “that Day” is the Day of the Lord - YMMV)
2 Thessalonians 2:3-4 King James Version (KJV)
3 Let no man deceive you by any means: for that Day shall not come, except there come a falling away first, and that Man of Sin be revealed, the Son of Perdition;
4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
All of this makes post-millennialism a head scratcher for me.