Charles Ellicott Commentary Jeremiah 31:6

"For there shall be a day, that the watchmen upon the hills of Ephraim shall cry, Arise ye, and let us go up to Zion unto Jehovah our God." — Jeremiah 31:6 (ASV)

The watchmen upon the mount Ephraim shall cry ... — The special fact is given as the basis of the previous prediction. The two kingdoms should be united, and therefore the possession of the vineyards should be undisturbed. The city of Samaria stood on one of the mountains of Ephraim. The “watchmen” may be either the sentinels stationed in the towers of the city, or, more probably, those that were on the lookout for the first appearance of the crescent moon as the signal for the observance either of the Passover or the new-moon festival. What follows is almost decisive in favor of the latter view.

What is implied is that the rival worship in Bethel and in Dan, which had so long kept the ten tribes of Israel from the Temple at Jerusalem, should cease, and that from the mountains of Ephraim there should be heard the cry which, with a solitary exception in the reign of Hezekiah (2 Chronicles 30:11; 2 Chronicles 30:18), had not been heard for centuries—Let us go up to Zion. The long schism which had caused the ruin of the nation would at last be healed. Unity of worship, at once the basis and symbol of national unity, should be restored.