The first mention of The Day of Atonement was after the tragedy happening right after Aaron and his sons were officially ordained as priest of God and his holy temple. On that day, God officially moved into the temple and his presence manifested for all of Israel to see:
however this day turned to tragedy as some of Aaron’s sons disobeyed God and as a result were instantly burnt up by God’s holy fire killing them:
Leviticus 10:2
“So fire came out from the presence of the Lord and consumed them, and they died before the Lord.”
In response to this tragedy, he warned Aaron himself should never come back into the holy of holies whenever he wanted, or he would die like his disobedient sons:
Leviticus 16:1
“The Lord spoke to Moses after the death of the two sons of Aaron who died when they approached the Lord. The Lord said to Moses ‘Tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover.’”
The only way Aaron could ever enter into the Holy of Holies again was only on a specific day and for a very specific reason:
He could only enter the holy of holies on the 10th day of the 7th month on God’s calendar: and on this day he was to come to only make sacrificial atonement before God for himself, his family, all of Israel, his holy temple, and the holy of holies for all the sins that have polluted them through the year:
Leviticus 16:3-29
“This is how Aaron is to enter the Most Holy Place:…[when he is to] mak[e]…atonement for himself and for the people…. for the Most Holy Place, the tent of meeting and the altar,… On the tenth day of the seventh month…”
It was at this time, that God revealed this special day Aaron could come into the Holy of Holies was also to be another special holiday for Israel:
On this day when Aaron or his descendants were making sacrificial atonements to cleanse their sins and their God’s holy temple’s defilement from their sins, they were to all fast (called here ‘denying yourselves’) and to have a sabbath day of rest on this sacred day of atonement:
Leviticus 16:29-34
“This is to be a lasting ordinance for you: On the tenth day of the seventh month you must deny yourselves(fast) and not do any work(the day would be a sabbath)—whether native-born or a foreigner residing among you— because on this day atonement will be made for you, to cleanse you. Then, before the Lord, you will be clean from all your sins. It is a day of sabbath rest, and you must deny yourselves; it is a lasting ordinance. The priest who is anointed and ordained to succeed his father as high priest is to make atonement. He is to put on the sacred linen garments and make atonement for the Most Holy Place, for the tent of meeting and the altar, and for the priests and all the members of the community. This is to be a lasting ordinance for you: Atonement is to be made once a year for all the sins of the Israelites.”
With this, God decreed a new holiday for Israel:
The Day of Atonement
A day dedicated to making sacrificial atonements for the nation of Israel for all their sins committed that year so far in order to cleanse them from their sin before their God and clean his temple if it had been defiled by that sin in the year; after the priest made atonement for all of Israel the whole nation would be sin-free and once again be clean in order to have a close and personal relationship with their God.
On this day Aaron (or one of his future descendants) were to do this ceremony at God’s holy temple to make atonement and to be able to enter into the holy of holies without dying in order to stand before God himself and make atonement in the Holy of Holies for Israel:
He was to first get a bull, and then from Israel get a Ram, and two male goats then he was to bathe his body then then put on the sacred white linen garments:
Leviticus 16:3-5
“This is how Aaron is to enter the Most Holy Place: He must first bring a young bull for a sin offering and…From the Israelite community he is to take two male goats for a sin offering and a ram for a burnt offering. He is to [then] put on the sacred linen tunic,…These are sacred garments; so he must bathe himself with water before he puts them on….[then he is to put on]…[the] linen undergarments next to his body; he is to tie the linen sash around him and put on the linen turban. ”
These sacred linen garments were not the usual priestly garments Aaron was ordained with or was to wear in everyday service in God’s temple: they were special pure white clothes only to be worn on this special day to atone for Israel.
First sacrifice the bull as a sin offering for his own sins and his family’s sins: in order to properly make atonement for anything else he must first make atonement for his own sins and his family's sins:
Leviticus 16:6
“Aaron is to offer the bull for his own sin offering to make atonement for himself and his household.”
Leviticus 16:12-13
“He is to take a censer full of burning coals from the altar before the Lord and two handfuls of finely ground fragrant incense and take them behind the curtain. He is to put the incense on the fire before the Lord, and the smoke of the incense will conceal the atonement cover above the tablets of the covenant law, so that he will not die.
While he did this NO OTHER human Levite was to be inside the temple: ONLY Aaron or one of his descendants serving as high priest was allowed to be in the temple at this sacred time:
Leviticus 16:17
“No one is to be in the tent of meeting from the time Aaron goes in to make atonement in the Most Holy Place until he comes out, having made atonement for himself, his household and the whole community of Israel.”
Then once covering God’s Prescence with the smoke from the coals, he was to get the bull’s blood and enter the Holy of Holies and stand in front of the holy Ark of the Covenant and sprinkle the bull’s blood seven times on the top covering the top of the ark (and right below where God’s holy Prescence is):
Leviticus 16:14
“...He is to take some of the bull’s blood and with his finger sprinkle it on the front of the atonement cover; then he shall sprinkle some of it with his finger seven times before the atonement cover.”
He then will come out of the holy of holies and go to the entrance of the temple and take two goats to the front of the temple he is to randomly select one of the goats by lot to be either the goat dedicated to God or the scapegoat to be banished to the wilderness:
Leviticus 16:7-8
“Then he is to take the two goats and present them before the Lord at the entrance to the tent of meeting. He is to cast lots for the two goats—one lot for the Lord and the other for the scapegoat.”
The Goat of God gets sacrificed at the alter for the sins of Israel and its blood is taken to the holy of holies and sprinkled on the top and front of the ark of the covalent before God's Prescence: this blood from the sacrificed Goat officially atones for all of Israel's sins for the entire year by cleansing the temple from the definment for their sins:
Leviticus 16:9-16
“Aaron shall bring the goat whose lot falls to the Lord and sacrifice it for a sin offering. In this way he will make atonement for the Most Holy Place because of the uncleanness and rebellion of the Israelites, whatever their sins have been. "
The whole temple would be cleansed and atoned for from the sin and uncleanness of Israel (and all of Israel as well) then Aaron was to finally bring the scapegoat out:
Leviticus 16:20
“When Aaron has finished making atonement for the Most Holy Place, the tent of meeting and the altar, he shall bring forward the live goat."
The scapegoat of the wilderness gets the sins of all of Israel for that year put on it and carries the sins of Israel far away from the camp into the wilderness never to be seen again, He lays his hand on its head and confesses all of the sins of Israel for that year onto the goat (thus symbolically putting the burdens of all theirs sins on the goat), he then will get someone to walk the goat away from Israel deep into the wilderness to be banished into the wilderness with their sins forever.
Leviticus 16:10-22
“But the goat chosen by lot as the scapegoat shall be presented alive before the Lord to be used for making atonement by sending it into the wilderness as a scapegoat. He is to lay both hands on the head of the live goat and confess over it all the wickedness and rebellion of the Israelites—all their sins—and put them on the goat’s head. He shall send the goat away into the wilderness in the care of someone appointed for the task. The goat will carry on itself all their sins to a remote place; and the man shall release it in the wilderness.”
thus, with the goat of God sacrificed and its blood covering the sins of Israel, and their sins taken far away from Israel to have the goat bear the burden for them: Israel and the holy temple was officially atoned for all that year, and it was like nothing had ever happened between them and God to begin with.
So Moses told Aaron these instructions for The Day of Atonement: and on the tenth day of the seventh month; he and all the Isrealites celebrated the first Day of Atonement for them as a nation being clensed from all their sin:
Leviticus 16:34
“And it (The first day of atonement) was done, as the Lord commanded Moses.”
In this way, God could continue living with his people in his temple and Isreal ,as a nation, could be forgiven of their sins for that year. With Isreal forgiven and God's temple cleansed from his people's sin for that year, they could now celebrate the most wounderful time of the year before their God in pure joy in the grandest and longest fall festival of Tabernacles/Temples (comming soon).