https://en.wikipedia.org/wiki/Jamestown%2C_Virginia

In August 1619, the first recorded slaves from Africa to British North America arrived at present-day Old Point Comfort, near the Jamestown colony, on a British privateer ship flying a Dutch flag.

It was when enslaved African people were first brought to Jamestown. Exactly when the start is more broadly is kind of harder to say since some things are more disputed

The arrival of the first captives to the Jamestown Colony, in 1619, is often seen as the beginning of slavery in America—but enslaved Africans arrived in North America as early as the 1500s.

https://www.history.com/articles/american-slavery-before-jamestown-1619

It’s when England began colonizing the americas. Which is kind of arbitrary since there were slaves in the Americas before that and the founding of The United States of America was over 100 years later.