Numbers 9:2-7

“Tell the Israelites to celebrate the Passover at the prescribed time, at twilight on the fourteenth day of the first month. Be sure to follow all my decrees and regulations concerning this celebration.” So Moses told the people to celebrate the Passover in the wilderness of Sinai as twilight fell on the fourteenth day of the month. And they celebrated the festival there, just as the LORD had commanded Moses. But some of the men had been ceremonially defiled by touching a dead body, so they could not celebrate the Passover that day. They came to Moses and Aaron that day and said, “We have become ceremonially unclean by touching a dead body. But why should we be prevented from presenting the LORD’s offering at the proper time with the rest of the Israelites?”