Numbers 9:2-7

“Tell the Israelites to celebrate the Passover at the appointed time. That appointed time is the fourteenth day of this month at twilight; they must obey all the rules about it.” So Moses told the Israelites to celebrate the Passover, and they did; it was in the Desert of Sinai at twilight on the fourteenth day of the first month. The Israelites did everything just as the LORD commanded Moses. But some of the people could not celebrate the Passover on that day because they were unclean from touching a dead body. So they went to Moses and Aaron that day and said to Moses, “We are unclean because of touching a dead body. But why should we be kept from offering gifts to the LORD at this appointed time? Why can’t we join the other Israelites?”