Many secular Jews do not observe other holidays or traditions through the year (just like people of other religions. )BUT:
Wikipedia says it best:
As one of the most culturally significant Jewish holidays, Yom Kippur is observed by many secular Jews who may not observe other holidays. Many secular Jews attend synagogue on Yom Kippur—for many secular Jews the High Holidays are the only recurring times of the year in which they attend synagogue,—causing synagogue attendance to soar—and almost four-fifths fast.
It's difficult for me to believe that you can live in this country and not be aware of Yom Kippur.
There's a site that I visit sometimes that states "because if we don't say something they might think we don't care."