When Yaakov leaves the land of Israel in fear for his life from Esau, he comes to a place when night has fallen – and the Torah adds that it was nightfall “because the sun had set.” Why does the Torah have to point out that the sun has set – it is night-time, after all?
In hindsight, it is obvious to us that Yaakov, and not Esau, was going to be the father of all of Israel. But at the time, it could not have been obvious at all. Yaakov had been away from Israel, and had had no direct contact with Hashem for many years. Indeed, the blessing in which Hashem says that Yaakov’s descendants will inherit the land of Israel , does not come until after the events at Shechem.
Esau and Yaakov shared both parents (unlike Yitzchak and Yishmael), and so it must have been at least possible to Yaakov that he was meant to share the blessing, and the future of all of his descendants, with Esau.
And so the sun sets when Yaakov leaves the land of Israel, and the world is cast into doubt and foreboding. Esau has stayed in the land with his parents (and Chazal suggest Yitzchak considered bestowing the birthright on Esau), while Yaakov has left the land of Israel, to live with Lavan. The sun has set.
And the Torah does not use the word for “sun” from the time Yaakov leaves Israel until Yaakov comes back to Israel. Esau’s angel comes to wrestle with Yaakov upon his return. He is wrestling to determine the dominance of either Esau or Yaakov, in the future of the Jewish people. Yaakov does not yield the future, and indeed he refuses to make peace with Esau’s angel. In this time of darkness, both literal and poetic, Yaakov fights tooth and nail for an outright victory, to utterly reject Esau as having any role.
At that moment, when the angel is pinned, and the shadow, the doubt, about the future of the Jewish people has been lifted, the sun rises on Yaakov, and the destiny of the Jewish people has been resolved.