Yom Kippur (the Day of Atonement) is one of two Jewish High Holy Days. The first High Holy Day is Rosh Hashanah (the Jewish New Year). Yom Kippur falls ten days after Rosh Hashanah on the 10th of Tishrei, which is a Hebrew month that correlates with September-October on the secular calendar. The purpose of Yom Kippur is to bring about reconciliation between people and between individuals and God. According to Jewish tradition, it is also the day when God decides the fate of each human being.
Although Yom Kippur is an intense holiday it is nevertheless viewed as a happy day. Why? Because if one has observed the holiday properly by the end of Yom Kippur they will have made peace with others and with God.
Most of the holiday is spent in the synagogue, in prayer. In Orthodox synagogues, services begin early in the morning (8 or 9 AM) and continue until about 3 PM. People then usually go home for an afternoon nap and return around 5 or 6 PM for the afternoon and evening services, which continue until nightfall. The services end at nightfall, with the blowing of the tekiah gedolah, a long blast on the shofar. See Rosh Hashanah for more about the shofar and its characteristic blasts.
Yom Kippur is the holiest day of the year, when we are closest to God and to the essence of our souls. Yom Kippur means “Day of Atonement,” as the verse states, “For on this day He will forgive you, to purify you, that you be cleansed from all your sins before G0d
Indeed, although Yom Kippur is the most solemn day of the year, it is suffused with an undercurrent of joy; it is the joy of being immersed in the spirituality of the day and expresses confidence that G‑d will accept our repentance, forgive our sins, and seal our verdict for a year of life, health and happiness.