Numbers 9:6-11

But some of them could not celebrate the Passover on that day because they were ceremonially unclean on account of a dead body. So they came to Moses and Aaron that same day and said to Moses, “We have become unclean because of a dead body, but why should we be kept from presenting the LORD’s offering with the other Israelites at the appointed time?” Moses answered them, “Wait until I find out what the LORD commands concerning you.” Then the LORD said to Moses, “Tell the Israelites: ‘When any of you or your descendants are unclean because of a dead body or are away on a journey, they are still to celebrate the LORD’s Passover, but they are to do it on the fourteenth day of the second month at twilight. They are to eat the lamb, together with unleavened bread and bitter herbs.