God told Jeremiah He would never break His covenant (promises) He has made with His people. When day no longer follows night then the covenant would end. He also promised to never abandon His people.
19 Then this message came to Jeremiah from the Lord: 20 “This is what the Lord says: If you can break my covenant with the day and the night so that one does not follow the other, 21 only then will my covenant with my servant David be broken. Only then will he no longer have a descendant to reign on his throne. The same is true for my covenant with the Levitical priests who minister before me. 22 And as the stars of the sky cannot be counted and the sand on the seashore cannot be measured, so I will multiply the descendants of my servant David and the Levites who minister before me.”
25 But this is what the Lord says: I would no more reject my people than I would change my laws that govern night and day, earth and sky. 26 I will never abandon the descendants of Jacob or David, my servant, or change the plan that David’s descendants will rule the descendants of Abraham, Isaac, and Jacob. Instead, I will restore them to their land and have mercy on them.”