Yom Kippur is the Day of Atonement. It is considered the most important holiday on the Jewish calendar. It is the day when we ask God for forgiveness and it is our last chance to repent before God signs and seals our fate for the coming year.
Yom Kippur falls 10 days after Rosh Hashanah usually in September or October.
It is a day of fasting, where we do not drink water or eat any food for nearly 26 hours. We do not wash or apply lotions, we do not wear leather and we spend most of the day in synagogue.
The fast begins at sundown after the Seudah Hamafseket, the meal eaten right before the fast, but the day before the fast we are supposed to feast. Some have the custom to eat a large fish meal earlier in the day and some eat honey cake with their meal. Find a collection of Yom Kippur recipes here.
The seudah hamafseket is a festive meal and many have the custom to eat kreplach soup at this meal, there are kabalistic reasons for this custom. Aside from the customs it is most important that you eat well and drink a lot of water before the fast.
After the fast everyone comes home from synagogue and it is common for friends and family to gather together to enjoy a light meal.