According to tradition, it is on the 7th day of Passover that the Israelites crossed the Red Sea and as they crossed they sang the song Az Yashir, praising God for saving them. The midrash (the interpretation of the Bible) explains that when Adam was created he never thanked God. When Abraham was saved he never thanked God. When Isaac was saved from the sacrifice by Abraham, he never said thank you.
When Jacob was saved from Laban and from his brother Esau, he never said thank you. Instead, it was the Israelites at the Red Sea who, for the first time in history, expressed their thanks to God. Thank you for rescuing us. And God said, I've been waiting for that. That's what the seventh day of Passover is about. Acknowledging the goodness of God, of people and saying thank you.