For Israel and the Jewish people, Yom Kippur (the Day of Atonement), began on the evening of Friday, September 29th and ends on the evening of Saturday, September 30th.
Yom Kippur—the Day of Atonement—is considered the most important holiday in the Jewish faith. Falling in the month of Tishrei (September or October in the Gregorian calendar), it marks the culmination of the 10 Days of Awe, a period of introspection and repentance that follows Rosh Hashanah, the Jewish New Year.
The 10th day of Tishrei (in 2017, from several minutes before sunset on Sept 29 until after nightfall on September 30), coming on the heels of Rosh Hashanah (the Jewish New Year, which is on the first and second days of Tishrei)
For nearly 26 hours we “afflict our souls”: we abstain from food and drink, do not wash or apply lotions or creams, do not wear leather footwear, and abstain from marital relations. Instead, we spend the day in synagogue, praying for forgiveness.

History of Yom Kippur

Just months after the people of Israel left Egypt in the year 2448 from creation (1313 BCE), they sinned by worshipping a golden calf. Moses ascended Mount Sinai and prayed to G‑d to forgive them. After two 40-day stints on the mountain, full Divine favor was obtained. The day Moses came down the mountain (the 10th of Tishrei) was to be known forevermore as the Day of Atonement—Yom Kippur.
That year, the people built the Tabernacle, a portable home for G‑d. The Tabernacle was a center for prayers and sacrificial offerings. The service in the Tabernacle climaxed on Yom Kippur, when the High Priest would perform a specially prescribed service. Highlights of this service included offering incense in the Holy of Holies (where the ark was housed) and the lottery with two goats—one of which was brought as a sacrifice, the other being sent out to the wilderness (Azazel).
While the High Priest generally wore ornate golden clothing, on Yom Kippur, he would immerse in a mikvah and don plain white garments to perform this service.
This practice continued for hundreds of years, throughout the time of the first Temple in Jerusalem, which was built by Solomon, and the second Temple, which was built by Ezra. Jews from all over would gather in the Temple to experience the sacred sight of the High Priest performing his service, obtaining forgiveness for all of Israel.
When the second Temple was destroyed in the year 3830 from creation (70 CE), the Yom Kippur service continued. Instead of a High Priest bringing the sacrifices in Jerusalem, every single Jew performs the Yom Kippur service in the temple of his or her heart.
The Book of Jonah is read in the synagogue on the afternoon of Yom Kippur, the holiest day in the Jewish calendar, the sacred Day of Atonement. Why, of all books in the Bible, this book this most holy day?
The answer is clear. The major themes of the book are singularly appropriate to the occasion—sin and divine judgment, repentance and divine forgiveness.
What is remarkable is that the work is not at all about Israel. The sinners and penitents and the sympathetic characters are all pagans, while the anti-hero, the one who misunderstands the true nature of the one God, is none other than the Hebrew prophet. He is the one whom God must teach a lesson in compassion.
It is precisely these aspects of this sublime prophetic allegory, and in particular the subthemes of the book, that inform Yom Kippur. These motifs attracted the ancient Jewish sages and led them to select Jonah as one of the day’s two prophetic lectionaries.  Its universalistic outlook; its definition of sin as predominantly moral sin; its teaching of human responsibility and accountability; its apprehension that true repentance is determined by deeds and established by transformation of character (Jonah 3:10), not by the recitation of formulas, however fervent; its emphasis on the infinite preciousness of all living things in the sight of God (Jonah 4:10–11); and, finally, its understanding of God as “compassionate and gracious, slow to anger, abounding in loving-kindness” (Jonah 4:2)—all these noble ideas of the Book of Jonah constitute the fundamentals of Judaism and the quintessence of Yom Kippur.
Written by: Biblical Archaeology Review & Chabad. org