Why was the first Passover in the promised land exactly 40 years after the Exodus Passover?
The Passover Israel celebrated upon entering the promised land fell exactly forty years after the Exodus Passover because Israel's unbelief at Kadesh caused God to turn them back into the wilderness for forty years of wandering. They could have entered much sooner, but God's meticulous timing brought them across Jordan to observe Passover on that exact anniversary.
Les teaches that Israel remained intact despite Moabite opposition and was about to cross Jordan and occupy the promised land. They would not have the easy entry they could have had at Kadesh; from Sinai to Kadesh they could have gone right in without losing a single young son or lifting a sword. But because of their unbelief—they did not believe God meant what He said—God in His wrath turned them back into the wilderness for forty years of wandering. Now, almost forty years later, as they cross the Jordan and get on the other side, they will stop for Passover. Les points out that this Passover, celebrated just after coming into the land, will be exactly forty years from the Passover in Egypt, showing how meticulous God is with His timing. Deuteronomy was written in the final month or two while they sat opposite the Jordan, as Moses reviewed everything from the coming out of Egypt.