The Lord spoke to Moses in the wilderness of Sinai, in the first month of the second year after they had come out of the land of Egypt, saying:
Let the Israelites keep the passover at its appointed time.
On the fourteenth day of this month, at twilight, you shall keep it at its appointed time; according to all its statutes and all its regulations you shall keep it.
References for Numbers 9:3
So Moses told the Israelites that they should keep the passover.
They kept the passover in the first month, on the fourteenth day of the month, at twilight, in the wilderness of Sinai. Just as the Lord had commanded Moses, so the Israelites did.
References for Numbers 9:5
Now there were certain people who were unclean through touching a corpse, so that they could not keep the passover on that day. They came before Moses and Aaron on that day,
and said to him, "Although we are unclean through touching a corpse, why must we be kept from presenting the Lord's offering at its appointed time among the Israelites?"
Moses spoke to them, "Wait, so that I may hear what the Lord will command concerning you."
The Lord spoke to Moses, saying:
Speak to the Israelites, saying: Anyone of you or your descendants who is unclean through touching a corpse, or is away on a journey, shall still keep the passover to the Lord.
In the second month on the fourteenth day, at twilight, they shall keep it; they shall eat it with unleavened bread and bitter herbs.
References for Numbers 9:11
They shall leave none of it until morning, nor break a bone of it; according to all the statute for the passover they shall keep it.
But anyone who is clean and is not on a journey, and yet refrains from keeping the passover, shall be cut off from the people for not presenting the Lord's offering at its appointed time; such a one shall bear the consequences for the sin.
Any alien residing among you who wishes to keep the passover to the Lord shall do so according to the statute of the passover and according to its regulation; you shall have one statute for both the resident alien and the native.