Charles Ellicott Commentary Numbers 9:5

"And they kept the passover in the first [month], on the fourteenth day of the month, at even, in the wilderness of Sinai: according to all that Jehovah commanded Moses, so did the children of Israel." — Numbers 9:5 (ASV)

And they kept the passover on the fourteenth day ... — There is no mention here of the observance of the feast of unleavened bread for seven days, as it was commanded in Exodus 12:18. It might not have been practicable to obtain a sufficient quantity of flour to last so large a number of people for seven days, though it may have been easy to procure from Midian or elsewhere a sufficient quantity for one meal.