You can read about this feast in the third book of Moses, among other things the following Lev. 16.29; 23.26: On the tenth day of the seventh month is the Day of Atonement. On this day you shall observe a holy gathering, and you shall mortify your souls, and you shall do no service on that day. Because on that day he will make atonement for you to cleanse you. You shall be cleansed of all your sins before the Lord. On the evening of the ninth day of the same month, until the next day evening you shall keep it as a holy day. Therefore the Jews gather in shul on the ninth day, before it becomes night. They carry their wax candles with them, put them up and light them, and they start to sing and pray with very unusual force. The women light special candles at home too, in all rooms, and in some places say a prayer over them or a Berácha, spread their hands over the light and separate the work-days from the holy days, as previously reported for their Sabbath. If the lights burn bright and well, they consider it as a good sign that their sins will be forgiven, or they will have good luck and not die. But if they burn poorly, melt or run, they consider it a bad sign, and are sad. They also spread rugs on the stone floor (in some places such as Worms they just spread grass on the hearth) so that the next day, when they prostrate themselves, or lie down on the hearth, they will not ruin their good clothes, or give an appearance of idolatry, which is prohibited, because it is said Lev. 26.1: You shall make no flagstones in your lands on which to bow down and prostrate yourselves.
It is also said that they should mortify, humble or torment their body, and they understand this to mean that they should abstain from five things. First, they should fast from one night to the other (it usually lasts twenty-four hours) including boys over twelve, girls over eleven, and new mothers, if the baby is more than three days old. A sick person, if asking for food, may eat, or if not asking but to follow doctor's orders, he or she may also eat. Another thing is that they may wear no shoes, but old people or those who suffer from the cold may wear socks or woolen slippers, or they may stand on a pillow in shul. Third, nobody shall put on scented oil or water for his pleasure. Fourth, nobody shall take a bath, or even put a little finger in water, much less wash his face or mouth in the morning. However if one has done his tzórech, answered the call of nature, he may put his fingers into water up to the knuckles. Some take a wet towel and wash their hands with it, but there is Saccanah, danger therein, because it might be too wet and a drop of water might be squeezed by hand which would be apparent work, and the holy day would be desecrated. Fifth, the men should keep away from their wives, and not touch them, just as if they were [menstrually] unclean.
Before they start their prayers that evening, three of the most prominent Rabbis go around the church and say in a loud voice: Bischibhah schel mahelah, ubischibha schel mattah etc. which means that they call upon, and publicly allow the whole congregation to pray together, the godless as well as the pious. To facilitate this, the cantor steps up to the Ark, in which the holy book of the law is kept, opens the Ark, and sings a long and devout prayer which starts: Col nidre vaessare uschebhue etc. that is, All vows, asseverations and oaths etc. [shall be void] and he sings the first part of this long chant three times in succession, each time higher and more joyfully. The meaning of the prayer is that all vows, oaths and promises, truth and faith, which a Jew has not kept throughout the year, shall be dissolved and annulled, not considered a sin, but held to be nothing, dissolved and forgiven. Therefore the pious and the truthful, along with the perjurers and the breakers of vows and promises, are joined together into a holy congregation, which may pray and praise God together. (Observe how much a Christian can rely on the oath of a Jew.) After that they continue until late at night. Some stay and pray all night in shul. Some go home and sleep, some sleep in a corner of the church, distant from the Ark, or in the church for the women [i.e. the women's gallery] after they all have left. Some of them who wish to be very pious, and repent fully, stay over the whole feast on their feet, day and night, praying continuously. I have seen some that stood for twenty-seven hours in one place.
Towards morning, before day breaks, they all gather together again in church, and stay together all day. They have many ceremonies with the Book of the Law. They fall often to the ground with their faces covered, especially when they say their confession, and they beat their breast with every word and show great devotion.
When it is almost night again, the priest puts his Talles (the big cloth which is wound around his neck) over his head, pulls it in front of his eyes, and gives the people the regular blessing, as is commanded in the fourth book of Moses Num. 6.22. While he says the blessing he spreads his hands out towards the people, but the people hold their hands before their eyes, and cover their face, because they are not allowed to look at the hands of the priest, because the spirit of God lies on the priest's hands when he says the blessing, as it is written Song 2.9: See, he stands behind our wall, sees through the window, and looks through the grating. That is, God stands behind the priest, and looks through the window and the grating, namely, through the spread-out fingers of the priest. After that they sing another prayer seven times in succession, each time more loudly, and they write that with this, God's majesty moves up towards the seventh heaven, and they accompany him with a beautiful Nigun, a lovely melody, which they sing as joyfully as they know how, as anyone who has ever heard it knows.
Before they leave the church they produce a loud and long sound from the previously mentioned horn in remembrance of the jubilee year which has started on that day. Others say it is in remembrance of the seven heavens that God opened when he gave the people of Israel the Law, showing them that in all the heavens there are no gods apart from him.
When they are all through, a voice comes from heaven (they write) and calls Eccles. 9.7: Go hence, eat your bread with joy, because God has accepted for good all your works. After that they go home, some of them take their left-over candles with them, and make habhdalah at home with them, that is they separate the holy day from the working days. Some of them leave the candles in church all year, and light them at special times. Some holy Jews burn a light day and night in their church throughout the year, which they call Ner tamid. And each one says to the other: Der Bore chasme dich leschanah tobhah, that is, May God the Creator seal you for a good year. As previously mentioned, the books will be sealed now, and after that, the sentence will not be changed.
When they come home they are all very hungry because they have eaten nothing in the last twenty-eight hours. So they sit down promptly at the table, and have a good evening meal, to make up for what they missed on that day.
However, the next day they get up early in the morning, and go again to shul, so that Satan may not start to complain and say: See, yesterday they got up early because it was atonement-day, but today their piety is over, they lie in bed and sleep long.
In sum, on this day they are so holy and pious, that even the devil himself has to praise their piety, like the statement you can read in Pirke Rabbi Eliezer, 46: On the day on which God gave the Thorah, or Law, the evil spirit Sammaël said to God: Lord of the whole world, you gave me power over all the peoples of the world, but over the people of Israel you have given me no power. God's answer: See, you shall have power over them on atonement-day, if you find any sins in them; but if you find no sins in them you shall have no power over them. (Therefore they give the evil spirit a gift on atonement day, so that he may not hinder the offering of the Israelites.) When the evil spirit Sammaël saw that the people of Israel were without sin on that day, he said to God: You have a people on earth that is just like the angels in heaven. Just as the angels stand up without food or drink, pure from sin, living in peace and unity, so do the people of Israel on atonement-day. When God hears this from the bad angel, then he forgives them all their sins and hears all their prayers. Elsewhere you read Caphtor upherach, p. 71: They give gifts to Satan to blind his eyes, so that he will not accuse them, as it is written Ex. 23.8: Gifts make blind those who can see.