The phrase "the last days" as it relates to the timeline of prophecy, occurs exactly seven times in the King James Bible. Jesus directly discusses the End Time and the events that both lead up to it and are in it in Matthew 24 and Mark 13.
The last days (Great Tribulation) is a period of roughly 3 1/2 years or 42 months (Revelation 11:2 - 3) that will shortly come upon the entire earth. It will a time, just before Jesus' Second Coming, when the Beast and the False Prophet will rule the world through their counterfeit version of God's kingdom.
The 42 months (see Revelation 11:3, 12:6) of the last or final period before Jesus return can further be divided into two relatively distinct pieces. The first piece is about two years (24 months) in length. The second or last piece, the "Day of the Lord" (Isaiah 2:12, Joel 2:11, 2Peter 3:10), is about 18 months in duration. This second part of the great tribulation represents God's direct punishment of unrepentant humanity.
The Apostle Paul warned his friend Timothy about the period before the return of Christ when he wrote, "Know this also, that in the last days perilous times shall come" (2Timothy 3:1, HBFV unless stated). Peter also warned the church regarding the state of the world near its end (2Peter 3:3).
Preparing the World
The Bible lists several events which will gradually lead to the great tribulation period. Humanity will increasingly adopt a negative attitude toward God's laws and reject keeping them. This lawlessness will cause people to greatly lack mercy, forgiveness and love (Matthew 24:9 - 10, 12).
The world will experience an increased frequency of wars, rumors of wars, earthquakes, famines and still more calamities (Matthew 24:6 - 7). Many false prophets will appear and deceive the masses with false hopes (Matthew 24:4 - 5, 11, 26 - 27). Deception, in general, will be rampant.
Many who considered themselves Christians and believers in the true God will also abandon their beliefs to avoid the trials of this last period (2Thessalonians 2:1 - 3).
A temple will be built in Jerusalem just before the public appearance of the Beast, False Prophet and the Two Witnesses. It will be in the temple that the "abomination of desolation" will declare that he is god in the flesh and demand he be worshipped (Matthew 24:15, 2Thessalonians 2:1 - 4, Revelation 11:1 - 3, 13:8, 12, 15).
Why Is It So Bad?
Why are the last times the most difficult period humanity will ever experience (Matthew 24:21)? One reason is that it will force ALL humans to choose one of two options - both of which bring pain and suffering.
Choosing to obey and worship the Beast during the last days will forestall the devil's wrath and allow people to live a little bit longer on earth. It comes, however, at the tremendous expense of experiencing horrific punishments from God (Revelation 14:9 - 10).
Choosing to obey the Eternal, however, will spare people God's wrath (see Revelation 9:4, 18:4). They will, however, be considered enemies of the world and martyred (Revelation 6:11, 7:14, 13:7 - 10, 15).
Last Attack of Heaven
Satan the devil will launch his last offensive in heaven in the hope he can wrest control of the universe away from God. He and his evil army will, however, be defeated by righteous angels. They will be cast back down to earth and permanently banned from coming back (Revelation 12:7 - 10).
A unique paradox will exist during the time of Satan's visible rule on earth. On the one hand, Christians will be persecuted and killed in great numbers while all others will be made to serve his agenda. Yet, on the other hand, in the last days many will grow rich and prosperous.
Revealing of Satan
The Beast and the False Prophet begin their work of deceiving man and ruling the earth under the direct inspiration of the Devil (Revelation 13:3 - 15). God's Two Witnesses also begin their ministry which entails proclaiming God's truth and calling people to repent (Revelation 11:3 - 7).
Place of Protection
Some (but not all) Christians will be miraculously taken to an earthly "place of safety" for the duration of the Tribulation (Revelation 12:6, 14, 13:5). They will be protected from the horrors of Satan's world-ruling government. They will be kept alive in the last days to await the Lord's return (1Thessalonians 4:16 - 17, 1Corinthians 15:51 - 53).
Mark of the Beast
Soon after the Beast and False Prophet take control over the world, they demand all humans receive a "mark." Those who refuse to accept this mark will not only be unable to buy or sell anything, they will be hunted down for their disobedience and killed (Revelation 13:14 - 17).
Martyrdom of Saints
In spite of what many religious organizations teach, not all Christians are whisked away in some secret rapture in the last days to be protected from the devil's wrath.
The Bible clearly states that not only will true believers exist on the earth during this last period of human governments, many of them (most? all?) will reject the Beast's mark. This will make them targets for persecution, which will ultimately lead to their martyrdom (Matthew 24:9, Revelation 6:11, 12:11, 17, 13:7, 14:12 - 13).