God spoke to Moses in the Wilderness of Sinai in the first month of the second year after leaving Egypt:
"Have the People of Israel celebrate Passover at the set time.
Celebrate it on schedule, on the evening of the fourteenth day of this month, following all the rules and procedures."
Moses told the People of Israel to celebrate the Passover
and they did - in the Wilderness of Sinai at evening of the fourteenth day of the first month. The People of Israel did it all just as God had commanded Moses.
But some of them couldn't celebrate the Passover on the assigned day because they were ritually unclean on account of a corpse. So they presented themselves before Moses and Aaron on Passover
and told Moses, "We have become ritually unclean because of a corpse, but why should we be barred from bringing God's offering along with other Israelites on the day set for Passover?"
Moses said, "Give me some time; I'll find out what God says in your circumstances."
God spoke to Moses:
"Tell the People of Israel, If one or another of you is ritually unclean because of a corpse, or you happen to be off on a long trip, you may still celebrate God's Passover.
But celebrate it on the fourteenth day of the second month at evening. Eat the lamb together with unraised bread and bitter herbs.
Don't leave any of it until morning. Don't break any of its bones. Follow all the procedures.
"But a man who is ritually clean and is not off on a trip and still fails to celebrate the Passover must be cut off from his people because he did not present God's offering at the set time. That man will pay for his sin.
"Any foreigner living among you who wants to celebrate God's Passover is welcome to do it, but he must follow all the rules and procedures. The same procedures go for both foreigner and native-born." The Cloud