Jeremiah 31:31, New covenant. When does this new covenant take effect?

Yeshua initiated the new covenant at the Last Supper (Matt 26:28), and will finalize it as per this prophecy at his second coming. It will be finalized after YHVH has gathered (or redeemed, v. 11) all the families of Israel (v. 1; i.e., the houses of Ephraim or Israel­—loosely speaking, the Christians and Judah—loosely speaking, the Jewish people, vv. 9, 20, 27, 31) from the north country, the coasts of the earth and the isles (vv. 8, 10) where they have been scattered, and brings them back to Zion (the land of Israel) with joy, singing and dancing (vv. 12–13, 24). Obviously, this hasn’t happened yet in its entirety. This will occur after Ephraim (Christians) repents (v. 20, of Torahlessness) and YHVH’s daughter turns away from her backsliding (vv. 21–22), and comes out of her captivity (v. 23, in spiritual Egypt or Babylon the Great, see Rev 18:4).

At that time, YHVH will make (or finalize) a new or renewed covenant with the two houses of Israel (vv. 31–33).

Also at this time, all Israel will know Elohim from the least to the greatest. Again, this prophecy has yet to be fulfilled. The writer of Hebrews quotes this verse in Hebrews 8:8. (See notes at Heb 8:8.)