Or HaChaim on Exodus 12:25/אור החיים על שמות י״ב:כ״ה

1 והיה כי תבאו אל הארץ, "It will be when you come to the land, etc." According to the plain meaning of the verse the commandment mentioned here does not apply until after the Israelites enter the Holy Land. If that were so, however, why did G'd appear to have changed His mind when He commanded the Israelites to observe the Passover in the second year of their wanderings in the desert "at the appropriate time and in all its details" (Numbers 9,1)?

2It would appear that this verse contains an assurance that if the Israelites would conduct themselves meritoriously they would already observe the following year's Passover in their homeland. The delay occurred only because of the Israelites' disobedience and rebelliousness which commenced already before the completion of the Exodus at the Sea of Reeds (compare Psalms 106,7: "they rebelled at the sea, at the Sea of Reeds"). All these factors delayed the conquest of the Holy Land. When G'd saw that they were still in the desert He had to issue a command that they should observe the Passover even in the desert. Up to that point in the Book of Numbers they had only been commanded to observe the Passover once they arrived in the Holy Land. While it is true- as the prophet Isaiah has said- that G'd knows of all future developments in advance (Isaiah 41,4), He had held back with issuing instructions which would indicate that the trek through the desert would be beset with problems.