The first Passover
The first Passover was actually the beginning of the ultimate final curse (the 10th curse) on Egypt for holding Israel captive: when God himself would come to Egypt and kill all the Egyptian’s firstborn sons (and their animal’s firstborn sons as well):
Exodus 11:1-7
“Now the Lord had said to Moses, ‘I will bring one more plague on Pharaoh and on Egypt. After that, he will let you go from here, and when he does, he will drive you out completely….About midnight (12:00 A.M.) I will go throughout Egypt. Every firstborn son in Egypt will die, from the firstborn son of Pharaoh, who sits on the throne, to the firstborn son of the female slave, who is at her hand mill, and all the firstborn of the cattle as well. There will be loud wailing throughout Egypt—worse than there has ever been or ever will be again. But among the Israelites not a dog will bark at any person or animal.’”
God then told Moses (and his brother Aaron) the Israelites also had to do something before this final plague or they too would have their firstborn son (and their animal’s firstborns) killed by God:
They had to immediately select a flawless yearling male lamb/goat (that is one that's a year old: in human years this would make the sheep/goat roughly equivalent to a teenager and one that had perfect fur/skin and no obvious defomities) and kill it at the end of the day and take the slaughtered lamb’s/goat’s blood and paint it onto the tops and sides of their front doors of their houses:
Exodus 12:3-7
“Tell the whole community of Israel that…each man is to take a lamb for his family, one for each household….The animals you choose must be year-old males without defect, and you may take them from the sheep or the goats….all the members of the community of Israel must slaughter them at twilight. Then they are to take some of the blood and put it on the sides and tops of the doorframes of the houses…”
God then explains why Israel was told to do this:
That night when he passed through Egypt to kill all the firstborn sons, when he would see the lamb’s/goat’s blood on the doorframes he would know to spare this home and the animals owned by this home from murdering the firstborn sons there; because the blood was a sign that marked the home as his obedient and submissive Israelite people that listened to his servant Moses' instructions:
Exodus 12:12-13
“On that same night I will pass through Egypt and strike down every firstborn of both people and animals, and I will bring judgment on all the gods of Egypt.…The blood will be a sign for you on the houses where you are, and when I see the blood, I will pass over you. No destructive plague will touch you when I strike Egypt…”
Since this was a holy night when God himself would come and kill all the Egyptian’s firstborn sons and also since Israel was about to be let go and freed they were to not only stay inside the house that night to be spared by the passing over of God’s wrath on Egypt but to also be ready to leave that night because this night was a holy night of God passing over all Egypt in a final judgement of a horrible plague.
So they were to not only pack up everything they wanted to take with them but they were to also then cook the slaughtered lamb/goat over a fire as quickly as possible on that night and to cook it with bitter herbs and eat it with flat bread with no yeast added and bitter herbs as soon as possible (burning all leftovers that remained that night they didn’t eat) to get energy to be prepared to be freed and take all their possessions with them out of Egypt: this night was to be known as God’s Passover!
Exodus 12:8-11
“...That same night they are to eat the meat roasted over the fire, along with bitter herbs, and bread made without yeast. Do not eat the meat raw or boiled in water, but roast it over a fire—with the head, legs and internal organs. Do not leave any of it till morning; if some is left till morning, you must burn it. This is how you are to eat it: with your cloak tucked into your belt, your sandals on your feet and your staff in your hand (in a position ready to leave any moment).Eat it in haste; it is the Lord’s Passover.”
So, Moses immediately ordered Israel to select the yearling male sheep/goats and get hyssop branches to paint the blood on their door frames that evening:
Exodus 12:21-22
“Then Moses summoned all the elders of Israel and said to them, ‘Go at once and select the animals for your families and slaughter the Passover lamb. Take a bunch of hyssop, dip it into the blood in the basin and put some of the blood on the top and on both sides of the doorframe.’ ”
Then he told them why he ordered them to do this:
Exodus 12:22-23
“None of you shall go out of the door of your house until morning. When the Lord goes through the land to strike down the Egyptians, he will see the blood on the top and sides of the doorframe and will pass over that doorway, and he will not permit the destroyer to enter your houses and strike you down.”
So, the elders told the other Israelites to also do the same thing, so they got the yearling male lambs/goats. Killed them at evening and took hyssop branches and painted their doorposts with their blood. Then they quickly roasted the lambs and as night came, went inside their houses and ate the lamb with unleavened bread and bitter herbs.
And at midnight that night, God came with the destroyer (possibly an angel) and passed over the houses of Israel that had the lamb’s/goat’s blood on the door that obeyed him but went into the enemy Egyptian houses and fields that did not have the lamb’s/goat’s blood on their doorposts and killed the firstborn sons of the Egyptian’s and the firstborn sons of their cattle in the field:
Exodus 12:29
“At midnight the Lord struck down all the firstborn in Egypt, from the firstborn of Pharaoh, who sat on the throne, to the firstborn of the prisoner, who was in the dungeon, and the firstborn of all the livestock as well.”
That very night after the destroyer had killed them, the Egyptian’s who were spared woke up around this time to find their firstborn son’s and firstborn cattle were dead and begin to cry in horror because at least one person/animal was dead in an Egyptian home!
Exodus 12:30
“Pharaoh and all his officials and all the Egyptians got up during the night, and there was loud wailing in Egypt, for there was not a house without someone dead.”
At this time, Pharaoh summoned Moses and Aaron and officially freed Israel from slavery to leave!
Exodus 12:31-32
“During the night Pharaoh summoned Moses and Aaron and said, ‘Up! Leave my people, you and the Israelites! Go, worship the Lord as you have requested. Take your flocks and herds, as you have said, and go.’”
Before and after that first Passover, God ordered ,from that day on, the anniversary of the night of Passover to be a holiday to be honored from now on by Israel:
After this holy night of judgement by God’s presence himself and the freedom of his people, God declared that this night be a holy holiday forever in honor of him coming down to fight for his people personally and their final freedom:
Exodus 12:14
“This is a day you are to commemorate; for the generations to come you shall celebrate it as a festival to the Lord—a lasting ordinance (a festival that will be celebrated always by Israel).”
Exodus 12:24-27
“Obey these instructions as a lasting ordinance for you and your descendants. When you enter the land that the Lord will give you as he promised, observe this ceremony. And when your children ask you, ‘What does this ceremony mean to you?’ then tell them, ‘It is the Passover sacrifice to the Lord, who passed over the houses of the Israelites in Egypt and spared our homes when he struck down the Egyptians.’”
The month of Abib/Nisan in which this event happened in, was to be the first month on the Israelite calendar:
Exodus 12:1-2
“The Lord said to Moses and Aaron in Egypt, ‘This month is to be for you the first month, the first month of your year.’”
Passover day happened on the 14th of the first Month (called Abib/Nisan) but the actual celebration with a special feast only happened in the later part of the day at late evening/early night: the time when God told Israel to prepare for his Passover that same time at night:
Leviticus 23:5
“The Lord’s Passover begins at twilight on the fourteenth day of the first month.”
On the 10th of Aviv (4 days before Passover), a perfect male lamb/goat a year old was to be bought and raised in the family home to be prepared for a sacrifice on Passover:
Exodus 12:3-5
“Tell the whole community of Israel that on the tenth day of this month each man is to take a lamb for his family, one for each household. ”
But then the lamb/goat bought 4 days earlier was to be killed at the beginning of the Passover (that is the early afternoon/evening of the 14th of Nisan/Abib):
Exodus 12:6
“Take care of them until the fourteenth day of the month, when all the members of the community of Israel must slaughter them at twilight.”
This was to officially mark the beginning of Passover and was done in honor of the anniversary of when the Israelites killed the original Passover lambs/goats and spread the killed lamb’s/goat’s blood on their doors in order to mark the human (and owned animal inhabitance) of the house/property as obedient and submissive to God (thus identified as obedient Israelites and not rebellious Egyptian enemies).
Also, at the beginning of Passover evening, Israelites were to stop eating yeast (all bread that had yeast in it).
Exodus 12:18
“...In the first month you are to eat bread made without yeast, ...[on]... the evening of the fourteenth day..."
This festival became the beggining of what would become the start of the spring festivals all celebrate in the first spring month of Abib (as well as the start of the entire festival season). Also, an ‘additional’ festival was connected by God to the original Passover: called the festival of unleavened bread (comming soon) that celebrated the freedom of Isreal from Egypt the next day and began right after this festival ended.