Matthew Poole and continuators on Numbers 9:1

1 AND the LORD spake unto Moses in the wilderness of Sinai, in the first month of the second year , after they were come out of the land of Egypt, saying,

And therefore before the numbring of the people, which was not till the second moneth, Numb. 1. 1, 2. But it is placed after it, because of a special case relating to the Passe-over, which hapned after it, and which is here related, upon occasion whereof he mentions the command of God for the keeping of the Passe-over in the Wilderness, which was done but once, and without this command they had not been obliged to keep it at all till they came to the land of Canaan. See Exod. 12. 25.