The Day of the Lord, Gospels and Epistles.
The Day of the Lord is a sure thing. The prophets of both Old and New Testaments describe the utter destruction of evil and everything attached to it. The letters to the Church describe the precursors. Jesus Himself said that Sodom and Gomorrah got off easy by comparison.
What is not sure is when it will happen. Indications of its approach include wars, famines, and earthquakes. Hatred, especially of Believers, will abound. People will be increasingly self-centered. False teachers who call themselves “Christian” will lead people astray and others will put on a false Christianity and do work to enhance their own reputations.
There have been multiple times in history that meet this description, and so it is impossible to say when of this or some future age will end with complete and final judgment. That day is known only by the Father- not even the angels know (Matthew 24:36). Christians are to stay awake and alert (Matthew 24, Luke 21, letters to the Thessalonians, Corinthians, 1 and 2 Peter). Beyond that, our mission is not to focus on future judgment, but on present ministry, living as authentic followers of Jesus who live out His love and work out our salvation by feeding the hungry, clothing the naked, visiting the sick and imprisoned, and welcoming strangers (Matthew 25).
This is to be our focus: loving the Lord with heart, mind, soul, and mind and loving our neighbors as ourselves (Matthew 22: 36-40). That is enough to keep us busy until the Day of the Lord comes.