What is the seventieth week of Daniel?
The seventieth week of Daniel is the final seven year period in which the antichrist makes a covenant with Israel, then breaks it halfway through to bring the abomination of desolation and the Great Tribulation.
The seventieth week of Daniel is a seven year period spoken of in Daniel 9:27, where the antichrist confirms a covenant with many for one week, meaning seven years. He will use this covenant to appear as Israel's protector, allowing the rebuilding of the temple and the resumption of sacrifices and worship, so that Israel will not oppose him during the first half of this time.
In the midst of the week, at the three and a half year mark, he causes the sacrifice and the oblation to cease. This is when he goes into the temple, sits on the mercy seat, and declares himself to be God, demanding worship. When Israel refuses, his fury turns against them, and he moves to destroy the city and the sanctuary. This second half of the week is the time of the abomination of desolation standing in the holy place, as Jesus warned in Matthew 24:15, quoting Daniel.
This final three and a half years, described as forty two months in Revelation 13:5, or as a time, times, and half a time in Daniel 7:25, Daniel 12:7, and Revelation 12:14, is the time of the Great Tribulation, a period of trouble such as has never been before. It is during this stretch that the faithful in Judea are told to flee into the mountains, for the days will be so severe that they must be shortened for the elect's sake.
A related time marker, 2,300 days, is given in Daniel 8:13-14 concerning how long the sanctuary and host will be trodden underfoot before the sanctuary is cleansed, which works out to about six years and four months, slightly less than the full seven year week. So while the antichrist's power to persecute in full fury lasts forty two months, his overall defiling presence in the temple and holy city spans the greater part of the seventieth week itself.
The scriptures
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 he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time.
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.
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.
And there was given unto him a mouth speaking great things and blasphemies; and power was given unto him to continue forty and two months.
And to the woman were given two wings of a great eagle, that she might fly into the wilderness, into her place, where she is nourished for a time, and times, and half a time, from the face of the serpent.
Then I heard one saint speaking, and another saint said unto that certain saint which spake, How long shall be the vision concerning the daily sacrifice, and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot? And he said unto me, Unto two thousand and three hundred days; then shall the sanctuary be cleansed.
When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:) Then let them which be in Judaea flee into the mountains: Let him which is on the housetop not come down to take any thing out of his house: Neither let him which is in the field return back to take his clothes. And woe unto them that are with child, and to them that give suck in those days! But pray ye that your flight be not in the winter, neither on the sabbath day: For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. And except those days should be shortened, there should no flesh be saved: but for the elect’s sake those days shall be shortened.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.