The great day of the LORD is near, it is near, and hasteth greatly, even the voice of the day of the LORD: the mighty man shall cry there bitterly. That day is a day of wrath, a day of trouble and distress, a day of wasteness and desolation, a day of darkness and gloominess, a day of clouds and thick darkness, A day of the trumpet and alarm against the fenced cities, and against the high towers. And I will bring distress upon men, that they shall walk like blind men, because they have sinned against the LORD: and their blood shall be poured out as dust, and their flesh as the dung. Neither their silver nor their gold shall be able to deliver them in the day of the LORD'S wrath; but the whole land shall be devoured by the fire of his jealousy: for he shall make even a speedy riddance of all them that dwell in the land. (Zep 1:14-18)

The last day will begin immediately after the Tribulation ends. Its beginning will be marked by powerful disturbances in the heavens, with the earth trembling in response:

Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken. (Mt 24:29)

The earth shall quake . . . the heavens shall tremble: the sun and the moon shall be dark, and the stars shall withdraw their shining . . . for the day of the LORD is great and very terrible; and who can abide it? (Joel 2:10-11)

But in those days, after that tribulation, the sun shall be darkened, and the moon shall not give her light, And the stars of heaven shall fall, and the powers that are in heaven shall be shaken. (Mk 13:24-25)

At the start of the last day, the powers of the heavens will be shaken. The ordained light-givers-the sun, moon and stars (Gen 1:14)-will darken, causing a reduction in earth's natural lighting. Also, stars will fall from the sky (not necessarily impacting the ground yet) and the earth will tremble, causing tidal disturbances. These events will take the world by surprise and people will be frightened:

And there shall be signs in the sun, and in the moon, and in the stars; and upon the earth distress of nations, with perplexity; the sea and the waves roaring; Men's hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken. (Lu 21:25-26)

Before the disturbances intensify too much, the Lord will return, like lightning flashing from one part of the sky to the other. Every person on earth will see Him, both believer and unbeliever alike: For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be . . . and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory. (Mt 24: 27, 30) Behold, he cometh with clouds; and every eye shall see him, and they also which pierced him: and all kindreds of the earth shall wail because of him. (Rev 1:7)

And then shall they see the Son of man coming in the clouds with great power and glory. And then shall he send his angels, and shall gather together his elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven. (Mk 13:26-27) When the Lord returns, He will gather all believers (the elect) to Himself. They will also receive their glorified bodies then, an event known as the "resurrection of life" (Jn 5:29). First to receive their glorified bodies will be the believing dead. Then, the believers who are alive at Christ's coming will be instantly transformed: