Numbers 9:3-7

On the fourteenth day of this month, at twilight, you shall celebrate it at its appointed time; you shall celebrate it in accordance with all its statutes and all its ordinances.”

So Moses told the sons of Israel to celebrate the Passover.

And they celebrated the Passover in the first month, on the fourteenth day of the month, at twilight, in the wilderness of Sinai; in accordance with everything that the Lord had commanded Moses, so the sons of Israel did.

But there were some men who were unclean because of contact with a dead person, so that they could not celebrate Passover on that day; and they came before Moses and Aaron on that day.

Those men said to him, “Though we are unclean because of a dead person, why are we kept from presenting the offering of the Lord at its appointed time among the sons of Israel?”