The curse took place sometime after the flood when Noah took up farming. After harvesting the grapes from his vineyard, he made wine and got drunk. His inebriated condition led him to remove all his clothes and take a nap on his tent floor (Genesis 9:20 - 21).
Ham, the youngest son who was at least 100 years old (see Genesis 5:32), then discovers his father Noah sleeping naked. He informs his two brothers who then discreetly cover him with a garment (Genesis 9:22 - 23). The patriarch then awakes, discovers what had been done to him by Canaan, and curses him for what he did!
And Noah awoke from his wine, and learned what his youngest grandson (Ham's son Canaan) had done to him. And he said, "Cursed be Canaan. He shall be a servant of servants to his brothers." (Genesis 9:24 - 25, HBFV throughout).
The Bible does not directly state what sin Canaan committed against Noah that earned him being cursed. It does offer, however, a few clues on what it might be.
Genesis 5 reveals how old eight of the nine patriarchs before Noah were at the time their firstborn son came into the world (verses 6 - 31). The Bible then takes great pains to inform us the nine patriarchs, from Adam to Lamech, had many more sons and daughters after their firstborn.
Noah had three sons around the time he turned 500 (Genesis 5:32, 11:10). Unlike all the other patriarchs, however, it does not state he produced any more children. This seems especially odd as he would have had plenty of time to procreate as he lived another 350 years after the flood (Genesis 9:28)!
Adding to the mystery of Noah's lack of a large family is the fact that both he and his sons were blessed by God after they left the ark. The Lord clearly wanted all four men to reproduce, not just Japheth, Shem and Ham. The Eternal, in fact, underlined his desire to have them procreate by admonishing them TWICE, unlike the single time he commanded Adam and Eve (Genesis 1:28), to reproduce!
And God blessed Noah and his sons, and He said to them, "Be fruitful and multiply, and replenish the earth . . . And you, be fruitful and multiply. Bring forth abundantly in the earth, and increase in it (Genesis 9:1, 7).
Noah was considered one of the most righteous humans to have ever lived (Ezekiel 14:14, 20). What could have stopped him from fulfilling God's command to have more children? What did Canaan do to him?
The Vile Act
Some believe that Noah's curse was caused by being sodomized by either Ham, Canaan or both. This, however, is unlikely, as this admittedly vile act would not have prohibited him from producing children.
It is highly probable that Canaan, who was a teenager or slightly older at the time, castrated his grandfather Noah and thus earned a curse. This act would have made it impossible for Noah to procreate further and obey the Lord. Canaan might have carried out such evil behavior out of spite or a desire to insure, as Ham's youngest son, less competition for blessings and a family inheritance.
Pattern of Sexual Sins
Sexual sins, perversion and idolatry would become some of the hallmark traits of Canaan and his descendants. This proves the curse he received from Noah was justified.
Canaan and his progeny lived in what would become the Promised Land and founded the cities of Sodom and Gomorrah (Genesis 10:15 - 19). The sins of these people became so base and gross that God personally carried out the death penalty upon them (Genesis 18:20 - 19:25). The evil influence of Canaan's descendants would also adversely affect Israel throughout its entire history (Deuteronomy 7:2 - 5, Ezra 9:1).