Yom HaKippurim (Day of Atonement) is the most solemn holy day of our year. On this day we lament and atone for the sins of B’nei Yisrael.
יהוה spoke to Moshe, saying: Mark, the tenth day of this seventh month is Yom HaKippurim. It shall be a sacred occasion for you: you shall practice self-denial, and you shall bring an offering by fire to יהוה; you shall do no work throughout that day. For it is Yom HaKippurim, on which expiation is made on your behalf before יהוה your God. (Vayikra 23:26-28)1
On this day, B’nei Yisrael is commanded to “afflict ourselves” which is often translated as “afflict your soul” however nefesh (נֶפֶשׁ) means soul and body. It has been understood that to “afflict ourselves” without violating the Torah means to fast for the entire day – evening to evening.2
The mortification required on that day [Yom HaKippurim] consists of abstention from food and drink, since all affliction of the soul mentioned in connection with the soul signifies fasting, as it is written: ‘I have afflicted my soul with fasting”, (Tehillim 35:13), and, ‘satisfy the afflicted soul’, (Yesha’yahu 58:10). (Aharon Ben-Eliyahu, Gan ‘Eden, Yom HaKippurim)2
There are very few Karaite customs for Yom HaKippurim. Special services are held at the Beit Knesset (Synagogue) where there are special penitent prayers offered (Vayikra 23:29). Food and drink are refrained from for the entire time between sunset and an hour after sunset the next night (Vayikra 23:32).2
1Stein, David (ed.). JPS Hebrew-English Tanakh. Philaelphia: Jewish Publication Society, 1999.
2al-Qirqisani Center. An Introduction to Karaite Judaism: History, Theology, Practice, and Custom. Troy, NY: al-Qirqisani Center for the Promotion of Karaite Studies, 2003.