Tribulation's darkest days begin when the Anti-Christ and the False Prophet appear in the form of Daval Angelino Blackmore and Dr. Simon Asher.
The people who missed the Rapture of Jesus Christ are devastated. They must now go through a global take-over, a one-world army, and a one-world currency. They can neither buy nor sell without the Mark of the Beast.666.
The people who turn to Christ during the first three and a half years will face being martyred. They along with the unsaved must face the four horseman of the book of Revelation which includes the opening of all seven seals and the woes and vials of God's wrath for three and half more years.
Rev 7:13-14
Then one of the elders answered saying to me, "Who are these arrayed in white robes, and where did they come from?" And I said to him, "Sir you know." So he said to me, "these are the ones who come out of the great tribulation, and washed their robes and made them white in the blood of the Lamb."