If an Israelite was unclean from a dead body at Passover, when could he observe it instead?

Answer: 14th day of the second month

Numbers 9 let those unclean from a dead body or away on a journey keep the Passover at twilight on the 14th day of the second month.

““Tell the Israelites: ‘When any one of you or your descendants is unclean because of a dead body, or is away on a journey, he may still observe the Passover to the LORD. Such people are to observe it at twilight on the fourteenth day of the second month. They are to eat the lamb, together with unleavened bread and bitter herbs;”— Numbers 9:10-11 (BSB)