Adam Clarke on Nehemiah 13:1

On that day - I am quite of Calmet's mind that the transaction detailed in this chapter did not immediately succeed the dedication of the walls of Jerusalem. It is most likely that, when this dedication was ended, Nehemiah returned to Babylon, as himself particularly marks, Neh 13:6, for he did return in the thirty-second year of Artaxerxes; and then, after certain days, supposed to be about the term of one year, he got leave to return to Jerusalem to see how matters were conducted: and there he found the evils which he mentions in this chapter, and which he redressed in the manner himself describes. See the introduction to this book.

Should not come into the congregation - That is, Ye shall not form any kind of matrimonial alliance with them. This, and this alone, is the meaning of the law.