“Yahweh said to Moses and Aaron in the land of Egypt, ‘This month shall mark for you the beginning of months. It shall be the first month of the year for you. Tell the whole congregation of Israel that on the tenth of this month, they are to take a lamb for each family, a lamb for each household.”
Here we have the priestly tradition of Passover. Yahweh said to both Moses and Aaron that this month would be the first month of the year. On the tenth of this month, each family would take a lamb. This assumes a settled society.
“If a household is too small for a whole lamb, it shall join its closest neighbor in obtaining one. The lamb shall be divided in proportion to the number of persons who eat of it. Your lamb shall be without blemish, a year-old male. You may also take it from the sheep or from the goats. You shall keep it until the fourteenth day of this month. Then the whole assembled congregation of Israel shall slaughter it at twilight. They shall take some of the blood and put it on the two doorposts and the lintel of the houses in which they eat it. They shall eat the lamb that same night. They shall eat it roasted over the fire with unleavened bread and bitter herbs. Do not eat any of it raw or boiled in water, but roasted over the fire, with its head, legs, and inner organs. You shall let none of it remain until the morning. Anything that remains until the morning you shall burn. This is how you shall eat it. Your loins girded and your sandals on your feet, and your staff in your hand. You shall eat it hurriedly.”
You also could join other families if your family was too small. You gathered on the tenth of the month but cook and eat it on the fourteenth day of this month at twilight. The lamb should be a one-year old unblemished male lamb. However, a sheep or goat may due. It is to be roasted, not boiled, and eaten the same night with unleavened bread and bitter herbs. Anything left over should be burned. You have to be ready to go. You have to put blood on the door posts to the house.
“It is the Passover of Yahweh. For I will pass through the land of Egypt that night, and I will strike down every first-born in the land of Egypt, both human beings and animals. On all the gods of Egypt I will execute judgments. I am Yahweh. The blood shall be a sign for you on the houses where you live. When I see the blood, I will pass over you, and no plague shall destroy you when I strike the land of Egypt. This day shall be a day of remembrance for you. You shall celebrate it as a festival to Yahweh. Throughout your generations you shall observe it as a perpetual ordinance.”
The Passover rules are very explicit, with no exceptions. Yahweh will pass over all the houses with the blood on the door posts. This Passover becomes the central perpetual memorial sacrificial feast for the Israelites because they became the unique people that they were becoming, the chosen ones.