All Israel Will Be Saved On That Day

Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. And in this way all Israel will be saved, as it is written, "The Deliverer will come from Zion, he will banish ungodliness from Jacob; and this will be my covenant with them when I take away their sins." (Romans 11:25-27 ESV)

All Israel will be saved when the fullness of the Gentiles (πλήρωμα τῶν ἐθνῶν) has come in. When will that happen?

Behold, a day is coming for the Lord, when the spoil taken from you will be divided in your midst. For I will gather all the nations against Jerusalem to battle, and the city shall be taken and the houses plundered and the women raped. Half of the city shall go out into exile, but the rest of the people shall not be cut off from the city. Then the Lord will go out and fight against those nations as when he fights on a day of battle. On that day his feet shall stand on the Mount of Olives that lies before Jerusalem on the east, and the Mount of Olives shall be split in two from east to west by a very wide valley, so that one half of the Mount shall move northward, and the other half southward. And you shall flee to the valley of my mountains, for the valley of the mountains shall reach to Azal. And you shall flee as you fled from the earthquake in the days of Uzziah king of Judah. Then the Lord my God will come, and all the holy ones with him.
On that day there shall be no light, cold, or frost. And there shall be a unique day, which is known to the Lord, neither day nor night, but at evening time there shall be light.
On that day living waters shall flow out from Jerusalem, half of them to the eastern sea and half of them to the western sea. It shall continue in summer as in winter.
And the Lord will be king over all the earth. On that day the Lord will be one and his name one. (Zechariah 14:1-9 ESV)

When Jesus comes again and lands on Mount Olives.

On that day I will make Jerusalem a heavy stone for all the peoples. All who lift it will surely hurt themselves. And all the nations of the earth will gather against it. On that day, declares the Lord, I will strike every horse with panic, and its rider with madness. But for the sake of the house of Judah I will keep my eyes open, when I strike every horse of the peoples with blindness. Then the clans of Judah shall say to themselves, 'The inhabitants of Jerusalem have strength through the Lord of hosts, their God.'
"On that day I will make the clans of Judah like a blazing pot in the midst of wood, like a flaming torch among sheaves. And they shall devour to the right and to the left all the surrounding peoples, while Jerusalem shall again be inhabited in its place, in Jerusalem.
"And the Lord will give salvation to the tents of Judah first, that the glory of the house of David and the glory of the inhabitants of Jerusalem may not surpass that of Judah. On that day the Lord will protect the inhabitants of Jerusalem, so that the feeblest among them on that day shall be like David, and the house of David shall be like God, like the angel of the Lord, going before them. And on that day I will seek to destroy all the nations that come against Jerusalem.
"And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and pleas for mercy, so that, when they look on me, on him whom they have pierced, they shall mourn for him, as one mourns for an only child, and weep bitterly over him, as one weeps over a firstborn. (Zechariah 12:3-10 ESV)