On that day - Or, “at that time,” as in Nehemiah 12:44.
The entire Pentateuch is probably meant by “the Book of Moses”.
Related Commentaries of Nehemiah 13:1
Nehemiah 13:1
1 On that day they read in the book of Moses in the audience of the people; and therein was found written, that the Ammonite and the Moabite should not come into the congregation of God for ever;