Well, I'm married - so I didn't vote in your poll, but I want to answer anyway! My husband and me celebrate three anniversaries. The first anniversary we celebrate is the day we met - Valentine's Day.
The second anniversary we celebrate is the day we started dating. This is also the day that my husband gave me my training collar (We live a BDSM lifestyle) so it is very special to us.
We also celebrate our wedding anniversary. There's pretty obvious reasons for celebrating that, but it is also the day I got my permanent locked on collar. It is very, very special to me.
We've been married for two years, and together for nearly 8, and we still celebrate these each year. (Though admittedly, the marriage thing is only for the past two).