Charles Ellicott Commentary Nehemiah 8:9

"And Nehemiah, who was the governor, and Ezra the priest the scribe, and the Levites that taught the people, said unto all the people, This day is holy unto Jehovah your God; mourn not, nor weep. For all the people wept, when they heard the words of the law." — Nehemiah 8:9 (ASV)

Mourn not, nor weep. —The days of high festival were unsuitable for public and, as it were, objective sorrow. The Day of Atonement was coming for that; as also the special day of fasting and covenant, which was already in the plan of Nehemiah and Ezra.