What was the Passover?
The Passover in the Hebrew Bible (i.e. the Christian Old Testament) was the annual Hebrew festival on the evening of the 14th day of the month of 'Abhibh (Abib) or Nisan, as it was called in later times. It was followed by, and closely connected with, a 7 days' festival of matstsoth, or unleavened bread, to which the name Passover was also applied by extension (Le 23:5).
Both were distinctly connected with the Exodus, which, according to tradition, they commemorate; the Passover being in imitation of the last meal in Egypt, eaten in preparation for the journey, while Yahweh, passing over the houses of the Hebrews, was slaying the firstborn of Egypt (Ex 12:12 f; Ex 13:2,12 ff); the matstsoth festival being in memory of the first days of the journey during which this bread of haste was eaten (Ex 12:14-20). The ordinance of peṣaḥ micrayim , the last meal in Egypt, included the following provisions:
(1) The taking of a lamb, or kid without blemish, for each household on the 10th of the month.
(2) The killing of the lamb on the 14th at even.
(3) The sprinkling of the blood on doorposts and lintels of the houses in which it was to be eaten.
(4) The roasting of the lamb with fire, its head with its legs and inwards - the lamb was not to be eaten raw nor sodden ( bāshal ) with water.
(5) The eating of unleavened bread and bitter herbs.
(6) Eating in haste, with loins girded, shoes on the feet, and staff in hand.
(7) And remaining in the house until the morning.
(8) The burning of all that remained; the Passover could be eaten only during the night
The Passover: Exodus 12:1-23
1 And the Lord spake unto Moses and Aaron in the land of Egypt saying, 2 This month shall be unto you the beginning of months: it shall be the first month of the year to you. 3 Speak ye unto all the congregation of Israel, saying, In the tenth day of this month they shall take to them every man a lamb, according to the house of their fathers, a lamb for an house: 4 And if the household be too little for the lamb, let him and his neighbour next unto his house take it according to the number of the souls; every man according to his eating shall make your count for the lamb. 5 Your lamb shall be without blemish, a male of the first year: ye shall take it out from the sheep, or from the goats: 6 And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening.
7 And they shall take of the blood, and strike it on the two side posts and on the upper door post of the houses, wherein they shall eat it. 8 And they shall eat the flesh in that night, roast with fire, and unleavened bread; and with bitter herbs they shall eat it. 9 Eat not of it raw, nor sodden at all with water, but roast with fire; his head with his legs, and with the purtenance thereof. 10 And ye shall let nothing of it remain until the morning; and that which remaineth of it until the morning ye shall burn with fire. 11 And thus shall ye eat it; with your loins girded, your shoes on your feet, and your staff in your hand; and ye shall eat it in haste: it is the Lord's passover.
12 For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the Lord. 13 And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt. 14 And this day shall be unto you for a memorial; and ye shall keep it a feast to the Lord throughout your generations; ye shall keep it a feast by an ordinance for ever.
15 Seven days shall ye eat unleavened bread; even the first day ye shall put away leaven out of your houses: for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from Israel. 16 And in the first day there shall be an holy convocation, and in the seventh day there shall be an holy convocation to you; no manner of work shall be done in them, save that which every man must eat, that only may be done of you. 17 And ye shall observe the feast of unleavened bread; for in this selfsame day have I brought your armies out of the land of Egypt: therefore shall ye observe this day in your generations by an ordinance for ever.
18 In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even. 19 Seven days shall there be no leaven found in your houses: for whosoever eateth that which is leavened, even that soul shall be cut off from the congregation of Israel, whether he be a stranger, or born in the land. 20 Ye shall eat nothing leavened; in all your habitations shall ye eat unleavened bread. 21 Then Moses called for all the elders of Israel, and said unto them, Draw out and take you a lamb according to your families, and kill the passover.
22 And ye shall take a bunch of hyssop, and dip it in the blood that is in the bason, and strike the lintel and the two side posts with the blood that is in the bason; and none of you shall go out at the door of his house until the morning. 23 For the Lord will pass through to smite the Egyptians; and when he seeth the blood upon the lintel, and on the two side posts, the Lord will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).