Previously, at the end of 1 Samuel 8, Samuel dismissed the elders of Israel (1 Samuel 8:22). Now he summons them back; only this time they will meet at Mizpah rather than Ramah (see 1 Samuel 8:4). Apart from being located at the top of a hill (and thus offering a good viewpoint), Mizpah is likely chosen because of its connection to the previous gathering that took place there (see 1 Samuel 7:1–17). When the people of Israel gathered there previously, it was to confess their sins before God and recommit themselves to the Lord. Likely Samuel is hoping that the new gathering will also result in some kind of confession and recommitment.
17 Now Samuel called the people together to the LORD at Mizpah.