Yom Kippur, otherwise known as The Day of Atonement (literally translated as such), is one of the most important religious holidays in Judaism. It occurs ten days after Rosh Hashanah. It is a day of intense reflection. Tradition states that the Book of Life is opened on Rosh Hashanah, whereas the Book of Life is "sealed" on Yom Kippur, sealing our fate for the year to come. This is a deeply reflective holiday, and a somber one at that. One would argue this holiday isn't "celebrated", but rather is observed due to the intense nature of the holiday. Starting at sundown to sundown, for those who are able we "afflict ourselves" by conducting a full fast. (We abstain from water and food as well as other things too. Such as wearing leather shoes, washing or bathing, applying lotion or creams, or sexual relations.) This begins a full 26 hours of fasting. (Those who are medically unable are advised to abstain from fasting.) Beginning at Dusk, a Kol Nidre service occurs. We use a book called a Mahzor to recite prayers from. Over the course of the holiday at synagogue you say the Viddui prayer approximately 5 times, which is the confession prayer. Unetanah Tokef is recited at the highest point of the holiday services, which is a religious poem that translated to "We shall ascribe holiness to this day". This is the only time that Jews prostate themselves during a prayer called the Avodah, meaning we literally go down on our knees and kneel. As well, there is a service called Yizkor memorial service, which is a way for the bereaved to honor to honor their deceased family publicly, instead of the usual practice of prayers being recited privately. It is some Jews tradition of those who have both living parents to leave this service as a sign of respect. The final prayer is the Neilah, in which the metaphorical heavenly gates are closed. Immediately after concluding of the services and upon sundown, the fast is typically broken as a community with a community meal. Typically you don't tell someone "Happy Yom Kippur" since it isn't really celebration, but rather an observance as described above. Instead its more appropriate and respectful to say "Have an easy fast" if you know the person is fasting, or if you aren't sure you can just say "Have a meaningful Yom Kippur." #GaviTalksJudaism Sources & further reading: https://www.myjewishlearning.com/article/yom-kippur/ https://www.chabad.org/library/article_cdo/aid/177886/jewish/What-Is-Yom-Kippur.htm https://www.myjewishlearning.com/article/unetanah-tokef/ https://www.myjewishlearning.com/article/understanding-viddui/ https://www.myjewishlearning.com/article/yizkor-the-memorial-service/