Numbers 9:4

And Moses spake unto the children of Israel, that they should keep the passover.

Numbers 9 resumes the narrative of Israel's journey from Sinai toward Kadesh-barnea, recording the people's experiences on the way. During that stay at Horeb or Sinai, in the year after deliverance from Egypt, God commanded Moses to tell the children of Israel to keep the Passover, and Moses did so. This was the first Passover they celebrated after leaving Egypt. Israel then observed it, but it was observed only once in the wilderness and was omitted afterward until the people entered Canaan.

The feasts given to Israel occurred annually and were to be observed only when due. The one exception was that those unable to keep the Passover in the first month on account of ceremonial uncleanness were instructed to keep it on the same day of the month in the second month.