Yom Kippur is considered a High Holiday for a handful of reasons. First, it is the most religiously concentrated day of the year; granted it is possible to go to Shacharit, Mincha, and Ma’ariv services any day of the week, but Yom Kippur has custom versions of each and other services and prayers besides. Second, it is motivated by a perceived level of risk; much of the praying is focused on asking for atonement for the wrongs of the previous year and for good fortune and health the year following. Third, it is a holiday where the primary focus is on G-d; while other holidays include feasting and recalling past triumphs and trials, Yom Kippur requires Jews to fast and focus on G-d alone.
I have gone to Yom Kippur services in a synagogue every year of my life, and at age 13 and beyond I fasted during the entire day. Since both my parents work as synagogue educators, I am used to spending the entire day at synagogue and in services.
The same thing occurred this year. I went to synagogue, I attended all the services from Kol Nidre through Neilah, and I fasted the entire time.