Yom Kippur atones for many sins, if we repent them. Therefore, Yom Kippur is a day of repentance. On Yom Kippur we are required to confess our sins to Hashem, but not to any human being.
When you commit a sin against Hashem, you must regret the sin, resolve never to do it again, and on Yom Kippur you must confess the sin and ask Hashem for forgiveness.
When you commit a sin that hurts another human being, you have sinned against both Hashem and that person. It is then necessary to add a step to the repentance process. You must regret the sin, resolve never to do it again, ask that person for forgiveness, and on Yom Kippur you must confess the sin and ask Hashem for forgiveness.
It is therefore customary to ask people before Yom Kippur to forgive you for anything you may have done to hurt them. If you make an honest attempt to ask the person for forgiveness, and he refuses to forgive you, you must try at least two more times. You have to wait a few days in-between requests, and they must be in three different places, in hopes that the person will cool off and change his mind. If he still refuses to forgive you, you have at least done your part, and Hashem takes that into account.
Likewise, if a person asks you for forgiveness, you should not be stubborn and refuse to forgive him. In general, Hashem treats us the way we treat other people, so it’s always good to remember that.
On Yom Kippur we are forgiven all our sins, and therefore we are absolutely pure, like angels. It is therefore the custom to wear white clothing, to appear like angels. It is also one of the many reasons we are forbidden to eat on Yom Kippur, since angels do not eat.
On Yom Kippur we fast, but it is forbidden to fast on Erev Yom Kippur — the day before Yom Kippur. In fact, Jewish Law states that we must feast on the day before Yom Kippur. The Talmud says that whoever feasts on the day before Yom Kippur is given merit and considered to have fasted two days!
Aside from breakfast, we eat two festive meals on Erev Yom Kippur. We eat one meal in the early afternoon or late morning, and another after Minchah (the afternoon services). That one extends until late afternoon, and must end no later than about fifteen minutes before sunset.
There is a custom among many people to eat kreplach on the day before Yom Kippur. Kreplach are bits of meat hidden in pockets of dough (sort of like won tons), and they are eaten in soup. The reason for this is that we hope and pray that G-d will hide our sins when we repent. (Eating kreplach is not a Law, by the way, so if you can’t eat them for some reason, never mind.)
There isn’t much to say about the customs of Yom Kippur itself. We spend the day in the synagogue praying, and all the instructions for that are in the prayer books. The best prayer book for the Holiday is probably the one put out by Artscroll. I use it myself. A good place to get it and other good books is Tiferes Stam. (Please note that I get no benefit at all from your shopping there. It just happens that I respect the owner of that store, and I consider him to be very honest and reliable.)
The prayer book for a Holiday is called a machzor, which means a “cycle.” Judaism teaches that the year is a repeating cycle in which the same holiness occurs each year and each week. Each and every Sabbath of the year, for example, has the same holiness and blessings of the very first
Sabbath of Creation. Each and every Yom Kippur of each and every year has the very same holiness as the very first Yom Kippur ever, the one observed by the Children of Israel in the Sinai Desert, 3,311 years ago. And each and every Yom Kippur of each and every year is just as conducive to our repentance being accepted.
Every day of the year we pray at least three main prayers. On Biblical Holidays, we pray four main prayers. On Yom Kippur we pray five main prayers.
The first is prayed the evening of Yom Kippur, and begins with Kol Nidrei. It continues with maariv, a slightly different version of that which we pray every night of the year.
The second is the morning Shacharis prayer. This too, has some slight differences, due to the Holiday.
The third is the Mussaf prayer, the “additional” service. Every Biblical Holiday has its own version of this prayer. Yom Kippur’s deals mostly with our repentance and our being forgiven.
The fourth is Mincha, the afternoon service. We pray Minchah every day, but of course the version prayed on any Holiday is very different than the version we pray every day.
The fifth and final prayer for Yom Kippur is unique to Yom Kippur, and is never prayed at any other time. It is called Ne’ilah, which means “closing.” It is prayed just as the sun begins to reach the tops of the trees, and the day is about to come to a close. (However, Yom Kippuer is not over until the sky is dark and the stars come out.) At that closing of the day, we wish to grab the last opportunity of attaining the greatness that this holy day offers us, by repenting fully before the day ends. Yes, you can repent any time and every time, but Yom Kippur grants us what few other days, if any, can grant us.
After Yom Kippur ends, we are required to recite or hear Havdalah over wine before we are allowed to eat anything. The Havdalah service declares the separation between the holy and mundane days, and Yom Kippur is the holiest day of the year.
As we leave the synagogue at the end of Yom Kippur, we bless each other with “A good and blessed year.”