To me my sobriety date is kind of like my birthday, I don't think about it every day but it is a significant date to me personally. I have also had two open heart surgeries and I remember both of those dates as well. When I was married me and my wife celebrated our wedding anniversary every year. I think most people like to remember and celebrate important times in their life by remembering the date that it occurred, and I look at my sobriety date as one of those times.
BTW I celebrate my sobriety date as April 17th, which was the first day I didn't take a drink from the time I woke up (I don't remember if I drank past midnight the day before???). On April 16th I went to my outpatient treatment assessment appointment (had 3 beers on the 30 minute drive there), and when I got home from the appointment I kept drinking until I went to sleep / passed out. I haven't had a drink since then.