In my 100 Things About Me post, I note that I am both a cultural Jew and an atheist (#81). I am reminded of what this means at this time of year, in part because people ask me about my decision to take the day off for Yom Kippur, and to fast. The problem with identifying one's self as a Jew is that the term has two meanings rolled into one word. When someone says s/he's an Irish Catholic, for example, that person is identifying both ethnicity/culture and religion. The term Jewish serves both functions, and as such, while I don't share the religious component, I do identify with the cultural. This is why I like to go to (or host) a Seder at Passover (and because it's a wonderful opportunity to be with family and/or friends).
That being said, what about Yom Kippur? Yom Kippur is the Day of Atonement, a fast day where Jews seek forgiveness from God and others for any misdeeds committed over the past year. The reason is to ensure one's inclusion in the Book of Life, i.e., the list of who will live for another year. I take the day off not to go to services, but to reflect. When people ask, I joke that I'll be contemplating my navel, but there's a lot of truth to that -- I think that taking one day each year to reflect on where I've been over the past year, and where I'm going over the next, is a constructive exercise, and for me, Yom Kippur seems like the logical day to do it.
As for fasting, it helps remind me what I should be doing during the day (i.e., helps me focus on the task at hand), and reminds me of what I have in the world compared with the hundreds of millions who lack necessary food and water every day, and for whom fasting a day here or there is often not a choice. It also helps me identify with my culture, sharing with Jews around the world this holy day that for me blurs the line between religion and culture. In addition, I have to acknowledge that there's a bit of pride that I have the discipline to fast (granted, it's a discipline that millions of others also possess). Lastly, it's habit (tradition?) -- I've been fasting on Yom Kippur for over 25 years, and I'm not ready to stop now.
L'Shana Tova Ha'Ba'ah -- May You and All Your Loved Ones Be Inscribed in the Book of Life