What does Mark 13:24 mean?

Jesus is warning Peter, James, John, and Andrew (Mark 13:3) about the end times. "That" tribulation speaks back to Mark 13:19 which says, "For in those days there will be such a tribulation…" The seven-year "tribulation" is broken into two parts. In the first three and one half years, the Antichrist will rise to power. In the last half, called the great tribulation, God will judge the world not only for its current rebellion, but for mankind's rejection of Him through the ages (Daniel 9:27). "That" tribulation means the great tribulation; so the event Jesus describes in Mark 13:24–27 takes place at the end of the tribulation, right before the millennial kingdom.

God established the sun, moon, and stars in creation for the purpose of being our source of light (Genesis 1:14–19). The removal of that light is always in the context of judgment. In Isaiah 13:10, the darkness accompanies—at least in metaphor— the ruin of the Babylonian Empire. Ezekiel 32:7 refers to the destruction of Egypt. When Jesus is crucified, the land goes dark for three hours (Mark 15:33). Isaiah 24:23 and Joel 2:10 reflect the prophecy Jesus gives here. First John 1:5 says that God is light. When light is taken away, so is God's blessing.

Before and during the tribulation, false teachers and liars claiming to be Christ will deceive people with counterfeit miracles (Mark 13:6, 22). Jesus' return will be much more powerful. Luke adds, "and on the earth distress of nations in perplexity because of the roaring of the sea and the waves, people fainting with fear and with foreboding of what is coming on the world" (Luke 21:25–26). Despite the warnings in God's Word and the testimonies of the 144,000 and the two witnesses (Revelation 7:4; 11:1–12), people on earth won't understand what is happening: judgment is upon them (Isaiah 34:1–2).