I celebrated the monthiversary when I was in high school, but have never done anything like that in my adult relationships...not that they're wrong, but they aren't me.
When DH and I were dating, we did have "the talk" around 6 months....but not on the exact date. And I think it was a good time. We celebrated our one year dating anniversary by getting engaged. But since neithr of us could remember the exact day we officially started dating, we just picked some random Saturday night and called it our anniversary. Since marriage, the only anniversary we celebrate is our wedding.
I think that as relationships mature (with time), then celebrating annually is the easiest fallback. For me, at this stage, it would be exhausting to celbrate every single month...or even every six months. More than just celebrating, it's too mentally exhausting for me to even think to simply acknowledge the day. Annually seems easier, more special, and for me, more appropriate. In high school, that was different. If I made it to six months with a guy I thought that was forever! My perspective has changed.
That being said, there are some that are way way more sentimental than me, so acknowledging a date isn't a bad thing. It's actually impressive that you can remember and remember to say something about it!