(Isa. 9:3)
In the Orient, the harvest time is always a time of great festivity. To the Jews of Bible days, it was also a time of great joy… . The law provided two feasts that were harvest festivals (Exod. 23:16). The first of these was called at one time The Feast of the Harvest, and later named The Feast of Pentecost. This feast was celebrated after the grain harvest… . The second of these feasts was sometimes called The Feast of Ingathering, being held after all the grain, fruit, wine, and oil had been gathered in… . It was also called the Feast of Tabernacles (Lev. 23:39–43), because they dwelt in booths to remind them of the wilderness days of the past.
(Fred H. Wight, Manners and Customs of Bible Lands [Chicago: Moody Press, 1953], 136–37.)
The entire process of harvesting caused much joy among the people, as it ensured them food for the coming year… . The Lord’s victory over Israel’s enemies and his coming will bring Israel a joy similar to that experienced by a farmer at an abundant harvest.
(Donald W. Parry, Visualizing Isaiah [Provo, Utah: The Foundation for Ancient Research and Mormon Studies, 2001], 78–79.)
In the King James Version this verse states that the people would NOT increase their joy, but the Book of Mormon gives the correct rendition. In fact, the King James translators inserted a marginal note indicating there was some question about the word “not.” The Revised Standard Version leaves out the “not” just as the Book of Mormon did nearly a century earlier. The word “not” obviously contradicts the next two phrases, which say that the joy of the people will be so exuberant that it will be similar to the happiness which always accompanies the gathering in of the harvest, or the happiness of those occasions when the booty is about to be distributed after a long, hard-fought campaign for victory.
(W. Cleon Skousen, Isaiah Speaks to Modern Times [Salt Lake City: Ensign Publishing Co., 1984], 222.)