Genesis 17:15

And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be.

The change from Sarai to Sarah was made when Abraham had been in Canaan nearly twenty-five years, when God appeared to him and commanded him to walk before God and be perfect. The new name Sarah means princess. It was given as a token of the fulfillment of the covenant that Abraham would become a father of many nations. With it came the promise that Sarah would be a mother of nations and that kings of peoples would come from her.