Even for the unobservant this is still one of the most important days in the Jewish calander. If there is no other day that a jew goes to synagogue/shul this is the day. It is a day of fasting and purifying where the jew seeks atonement for all the sins they have commited against G-d (and only those against G-d, sins committed against another person must be attoned for through seeking forgiveness from that person). It is a Sabbath day meaning that no work must be done at all.

there is a fasting that is done for 24 (sometimes 25) hours and most of the day is spent at synagogue (shul).

for more on this important day check out...
http://www.jewfaq.org/holiday4.htm
http://www.ou.org/chagim/yomkippur/default.htm

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