Yom Kippur is well-known as a fast day in Judaism. From sundown until sundown, we are not supposed to eat or drink anything at all (of course, exceptions are made for health). In fact, fasting is only the best-known of the Yom Kippur restrictions—there are actually five major ones:
- bathing (for pleasure, not for basic cleanliness)
- wearing leather shoes
- anointing (which was probably more common couple of thousand years ago than it is now, although, if you include lotions and such, then it's still pretty relevant to a lot of people)
Together, these five restrictions are supposed to constitute “afflicting the soul,” in the words of the Torah. Why do we do that? Well, as usual, the Torah doesn't tell us, explicitly, so we don't have one answer. At the simplest, most obvious level, it's a form of punishment—a kind of self-flagellation. This is our Day of Atonement, so we symbolically punish ourselves (or, I guess, not so symbolically) for all the things that we did wrong this past year.
The explanation which I remember most from my childhood revolved around learning to appreciate what we have, and being more sensitive to those who go without. Whenever I would complain about being hungry, it seems that someone would always say, “Just think about those who are this hungry every day.” Our own small act of sacrifice can remind us of how fortunate we are, and drive us to help those who are truly needy.
Fasting can also be, for some of us, a form of concentration. I remember being told, many times, that we don't eat on Yom Kippur because we don't have time—we are supposed to be so busy with our prayers, our personal reflection, and our teshuvah (repentance) that we can't even take a break to take care of ourselves. Even though that's probably never the literal truth (I doubt that anyone is really capable of focusing solely on those lofty topics for every waking moment of an entire day) it's still a valuable teaching. The work of Yom Kippur is substantial—it's not something we can complete in just a couple of hours. If we really want to do teshuvah, and we're going to have to seriously dedicate ourselves to it during the time leading up to the High Holy Days, and especially on those days, themselves.
We'll be able to eat when the sun goes down. Until then, we'll have more important things to be doing.