Note that this is not predicting Israel's "darkest day" but it is predicting the massive downfall of the oppressing power which will lead to the salvation of Bnai Yisrael. However, during that time it will appear to be a time of tribulation.
Reading the next pesukim Yirmiyah 30:8-9 we see that it can be the coming of the mashiach (which is why Rashi refers to the war of Gog and Magog as well as the beginning of the end of the 70 years of exile.
And it shall be on that day, says the Lord of Hosts, [that] I will
break his yoke off your neck, and I will break your thongs, and
strangers shall no longer enslave them.
Since the Bnai Yisrael did not merit complete redemption after the 70 years, we will have to apply it to the travails of the time of the Mashiach.
Abarbanel explains that since there will be an intense war and many casualties, the people of Israel will feel troubled because of the fear of the nations and the fighting. However, as a result of all this the redemption will result from the war. He explains that verse 11 shows that this is the coming of the Mashiach and not the fall of Bavel because of all the other nations mentioned then.
Radak also connects it to the war of Gog and Magog and the prophesies of Yeshaya 26:20 and Daniel 12:1
Malbim on Yirmiyahu 30:10
And you, fear not, My servant Jacob, says the Lord, and do not be
dismayed, O Israel, for behold I save you from afar and your seed from
the land of their captivity, and Jacob shall again be silent and at
ease, and no one will frighten them.
says like Rashi
And you, fear not: Because of all the evils that you will see befalling
Babylon in whose midst you are.
That the Bnai Yisrael should be careful not to become frightened at the destruction that they see all around them as it will lead to redemption (Cyrus).