John Gill Commentary Nehemiah 13:1

"On that day they read in the book of Moses in the audience of the people; and therein was found written, that an Ammonite and a Moabite should not enter into the assembly of God for ever," — Nehemiah 13:1 (ASV)

On that day Not when the wall of the city was dedicated, nor quickly after; for it cannot be thought that people should be so corrupted so soon as this chapter shows; but when Nehemiah had governed them twelve years, and had been at Babylon, and was returned again, as appears from (Nehemiah 13:6) , compared with (Nehemiah 2:1Nehemiah 2:6) (7:2) ,

they read in the book of Moses in the audience of the people ; for from the time of the reading of the law by Ezra, (Nehemiah 8:1) it became a custom to read the law publicly:

and therein was found written, that the Ammonite and the Moabite should not come into the congregation of the Lord ; that is, be admitted to marry with any of the people of Israel; (See Gill on Deuteronomy 23:3).