Numbers 9:4-6

New International Version - UK

⁴ So Moses told the Israelites to celebrate the Passover, ⁵ and they did so 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.

⁶ 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