Numbers 9:4-6

Amplified Bible

⁴ So Moses told the Israelites to observe the Passover. ⁵ They observed the Passover on the fourteenth day of the first month at twilight in the Wilderness of Sinai; in accordance with all that the Lord had commanded Moses, so the Israelites did. ⁶ But there were certain men who were [ceremonially] unclean because of [touching] the dead body of a man, so they could not observe the Passover on that day; so they came before Moses and Aaron that same day.