Why does the Lord say that if his covenant with the day and the night can be broken, then also his covenant with “David my servant may be broken” and his “covenant with the Levitical priests”?

 Much like he did in Jeremiah 31:35–37, the Lord compares his faithfulness to promises made in the past, to his faithfulness in sustaining creation. If you can break my covenant with the day and my covenant with the night, so that day and night will not come at their appointed time, is probably a reference to the promise that the Lord makes to Noah after the flood where the Lord affirms his commitment to his creation, and promises to not destroy his world because if its sin (Genesis 8:20–9:17)¹. As firm as his commitment is to life continuing in his world, so firm is the Lord’s commitment to the covenant he made with David regarding a son to reign on his throne (see 2 Samuel 7:1–17) and the promise he made to Phinehas regarding a permanent priesthood from the tribe of Levi (seeNumbers 25:10–13; Nehemiah 13:29; Malachi 2:4, Malachi 2:8)². So too is the Lord committed to the promise he made to Abraham that his descendants would outnumber the stars in the sky and the sand of the sea (see Genesis 13:16; Genesis 15:5; Genesis 22:17 etc.). The Lord’s plans and purposes as both Creator and Redeemer are inseparable. Sin and rebellion will not undermine his commitment to the world he made and his promises of redemption are as unshakable as his commitment to sustaining the life he created. These promises are ultimately fulfilled in Christ who is the promised Messiah (Mark 8:29) and the perfect High Priest (Hebrews 8:1–13) who brings into existence the Lord’s people, a great multitude from every tribe and nation (Revelation 7:9) who are themselves a royal priesthood (1 Peter 2:9; Revelation 1:6;Revelation 5:10) and who rule with Christ over creation as the Lord intended (Revelation 5:9–10).