It is the Day of Atonement and is the most holy day in the Jewish year. And it is the day when God decides what will happen to every Jewish person. It's not like the other special Jewish days as it doesn't remind us of Jewish history but instead it is about a Jewish persons behaviour.
Every person makes mistakes and yom kiper gives us the chance to ask for forgiveness from people for anything we have done or said to make them sad. On Yom kiper we ask God to forgive us and say we will try not to do it again.
Yom kiper is ten days after Rosh Hashone. It starts at sunset the day before Yom kiper and finishes the next day at sundown. People fast for all of this time.
Yom kiper is a day of prayer we go to shule to pray, to think about all the good things that we did and the not so good things that we did. We decide we won't do these bad things again.
On Yom kiper Jewish adults fast approximately 24hrs. They do not eat or drink,that includes children over Bar or Batmitzvah age (13 for boys & 12 for girls). Children over nine can fast for part of the day. Anyone who is sick is excused from fasting. It is a custom to wear white clothing on Yom kiper but you don't have to. The curtain and the covers across the Torah in shule is also white.
There is book about Yom kiper called K'Tonton's Yom Kippur Kitten by Sadie Rose Weilerstein
It is about K'ton who is a very tiny boy, the size of a thumb. He wants to fast but he gets very hungry he blames a small kitten for something he did. K'Tonon then feels guilty so he tells his mother what he did and asks the kitten for forgiveness.