To my Jewish readers: A Happy 5779, although I am told that the traditional greeting for Yom Kippur is "Have an easy fast".
The Jewish Year 5779 starts at sunset September 18, 2018
I took the following short excerpts about Yom Kippur from a longer and more explanatory article at Judaism 101
Significance: Day of Atonement
Observances: Fasting, Prayer and Repentance
Length: 25 Hours
Liturgy additions: Annulment of vows; lengthy confession of sins
Yom Kippur is probably the most important holiday of the Jewish year. Many Jews who do not observe any other Jewish custom will refrain from work, fast and/or attend synagogue services on this day. Yom Kippur occurs on the 10th day of Tishri. The holiday is instituted at Leviticus 16:29-30.
The name "Yom Kippur" means "Day of Atonement," and that pretty much explains what the holiday It is a day set aside to "afflict the soul," to atone for the sins of the past year.