Yom Kippur, which translates to "Day of Atonement" in Hebrew, is a Jewish holiday that focuses on repentance and atonement. It is the holiest day in Judaism and takes place on the 10th day of Tishrei, which is usually in late September or early October. 

https://en.wikipedia.org/wiki/Yom_Kippur

Here are some observances of Yom Kippur: 

Fasting: A 25-hour fast that is the only one prescribed in Jewish holy scriptures. Some restrictions can be lifted if there is a threat to life or health. 

Prayer: Jews attend synagogue for prayer and services. 

Repentance: Jews seek forgiveness from God and others for their wrongs. 

No work: Jews abstain from work. 

No eating or drinking: Jews refrain from eating or drinking any food or liquids. 

White clothing: It is customary to wear white, which symbolizes purity. Some people also wear a kittel, the white robe in which the dead are buried. 

The holiday begins at sundown with prayer and meditation. The services conclude with the blowing of the shofar, a ritual horn, which marks the joy and faith that celebrants have had their sins forgiven