The LORD spoke to Moses in the Desert of Sinai in the first month of the second year after they came out of Egypt. He said,
“Have the Israelites celebrate the Passover at the appointed time.
Celebrate it at the appointed time, at twilight on the fourteenth day of this month, in accordance with all its rules and regulations.”
So Moses told the Israelites to celebrate the Passover,
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.
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