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The most important holy day for the Israelites, also called Yom Kippur (from Hebrew yohm hak·kip·pu·rimʹ, “day of the coverings”), held on Ethanim 10. This was the only day of the year on which the high priest went into the Most Holy of the tabernacle. There he offered the blood of the sacrifices for his sins, the sins of the other Levites, and the sins of the people. It was a time of holy convention and fasting, and it was also a sabbath, a time to abstain from regular work.—Le 23:27, 28.
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Leviticus 23
27 Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the Lord.
28 And ye shall do no work in that same day: for it is a day of atonement, to make an atonement for you before the Lord your God.
It is not reliable to let people tell you what the bible says. Many people will make up stuff because they don't know what it says, and many will make up stuff because they wish it would not say what it says. You just have to read it for yourself. There are many sites where you can look up bible passages. I use www.biblegateway.com/passage/?search=day+atonement&version=KJV
Read a chapter of Proverbs every day. Proverbs has 31 chapters so you can keep your place by just looking at a calendar. There is no religion or nothing in Proverbs and you don't have to believe anything. Just read to find wisdom. When you are comfortable with that, then read the bible from Romans to 2 Thessalonians over and over until you start to remember what it says. That is the part that applies to Christians.