What is the time of Jacob’s trouble?
The time of Jacob's trouble is the period of intense distress that comes upon Israel and Jerusalem during the second half of the seven year covenant, when the antichrist breaks his pact, defiles the temple, and Jerusalem is surrounded and trodden down by armies.
The time of Jacob's trouble refers to the season of great distress that falls specifically on Israel and Jerusalem in the last half of the seven year covenant the antichrist makes with the nation. Jesus described it plainly: "And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh. Then let them which are in Judaea flee to the mountains... For these be the days of vengeance, that all things which are written may be fulfilled" (Luke 21:20-22). It is called Jacob's trouble because it is Israel being rewarded for her disobedience, a chastening that comes upon the covenant people rather than the church.
During this time there will be great distress in the land and wrath upon the people. "They shall fall by the edge of the sword, and shall be led away captive into all nations: and Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled" (Luke 21:23-24). This is tied to the covenant the antichrist confirms for one week (seven years); in the midst of that week he causes the sacrifice to cease and sets up the abomination that makes the temple desolate, turning against Israel and provoking the desolation described in Daniel 9:27.
Daniel also gives the length of this scattering of the holy people as "a time, times, and an half" (Daniel 12:7), the same period in which Israel is overtaken, scattered, and purged, until the appointed time of the end. So the time of Jacob's trouble is not a vague idea but a specific, measured season of siege, captivity, and chastening that falls on the Jewish nation in the latter half of the tribulation period, right up until the Lord returns to deliver the remnant.
The scriptures
And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh. Then let them which are in Judaea flee to the mountains; and let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto. For these be the days of vengeance, that all things which are written may be fulfilled. But woe unto them that are with child, and to them that give suck, in those days! for there shall be great distress in the land, and wrath upon this people. And they shall fall by the edge of the sword, and shall be led away captive into all nations: and Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled. 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.
And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate.
And one said to the man clothed in linen, which was upon the waters of the river, How long shall it be to the end of these wonders? And I heard the man clothed in linen, which was upon the waters of the river, when he held up his right hand and his left hand unto heaven, and sware by him that liveth for ever that it shall be for a time, times, and an half; and when he shall have accomplished to scatter the power of the holy people, all these things shall be finished.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.