In truth Jesus is telling them exactly which festival within that generation that it would happen to within a couple of days. Here's a paragraph from an article from:
No Man Knows The Day or The Hour | Hatikva Ministries
The seventh month,
Tishri, has three inter-linked festivals. The first festival occurs on the first day of the seventh month which of course would also be a
Rosh Chodesh or New Moon. This festival is generally called
Rosh haShanah which means the Head of the Year, and is commonly celebrated as the Jewish New Year. Among most Christians this festival is known as the Feast of Trumpets. This title somewhat misconveys the actual Biblical name of the festival,
Yom Teruah, which means the Day of the Awakening Blast. The next festival is
Yom Kippur, the Day of Atonement, and occurs on the 10th of
Tishri. Five days later the Festival of
Sukkot begins and is celebrated for 7 days from the 15th of
Tishri through the 21st. An additional day is attached to
Sukkot but is viewed as being separate from these 7 days. This day is known as
Shimini Atzeret, the Eighth Day and occurs on the 22nd of
Tishri. Therefore, once the Sanhedrin has set the
Rosh Chodesh for
Tishri, the rest of the festivals could be calculated.
Rosh haShanah stands alone during Tishri, as well as the rest of the year, as being the only festival of which no man knew the day or the hour. It could only be determined by the decree from the court declaring the the New Moon had been spotted.
Romans 8:
29For those whom he
foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers.
30And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified.
Romans 9:
3For I could wish that I myself were accursed and cut off from Christ for the sake of my brothers, my kinsmen according to the flesh.
Why would Paul say that if all kinsmen according to the flesh are going to be saved? The answer:
Romans 9:
6But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel,
7and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.”
8This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring.
Galatians 3:
28There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus.
29And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
Acts 3:
22Moses said, ‘The Lord God will raise up for you a prophet like me from your brothers. You shall listen to him in whatever he tells you.
23And it shall be that every soul who does not listen to that prophet shall be
destroyed from the people.’
24And all the prophets who have spoken, from Samuel and those who came after him, also
proclaimed these days.
destroyed:
exolethreuó: to destroy utterly
Original Word: ἐξολοθρεύω
Part of Speech: Verb
Transliteration: exolethreuó
Phonetic Spelling: (ex-ol-oth-ryoo'-o)
Definition: to destroy utterly
Usage: I destroy utterly, annihilate, exterminate, root out.
Those who didn't come to Christ would have no more inheritance to pass on.
Galatians 4:
21Tell me, you who desire to be under the law, do you not listen to the law?
22For it is written that Abraham had two sons, one by a slave woman and one by a free woman.
23But the son of the slave was born according to the flesh, while the son of the free woman was born through promise.
24Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar.
25Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children.
26But the Jerusalem above is free, and she is our mother.
27For it is written,
“Rejoice, O barren one who does not bear;
break forth and cry aloud, you who are not in labor!
For the children of the desolate one will be more
than those of the one who has a husband.”
28Now you, brothers, like Isaac, are children of promise.
29But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now.
30But what does the Scripture say? “Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman.”
31So, brothers, we are not children of the slave but of the free woman.
As you can see, the earthly Jerusalem is cast out. Superseded by the heavenly Jerusalem.
Hebrews 12:
22But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to innumerable angels in festal gathering,
23and to the assembly of the firstborn who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous made perfect,
24and to Jesus, the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel.