Genesis 21:4 (NIV) When his son Isaac was eight days old, Abraham circumcised him, as God commanded him.
Starting with Abraham, the Israelite men were circumcised, and boys at the eighth day after birth. It was commanded again in the desert through Moses:
Leviticus 12:3 (NIV) On the eighth day the boy is to be circumcised.
But the children born in the desert were not circumcised:
Joshua 5:5 (NIV) All the people that came out had been circumcised, but all the people born in the wilderness during the journey from Egypt had not.
How is this possible? Did they disobey God and not circumcise their children in the desert?