I think the celebration idea is a wonderful one.
I'm not a parent, but I really like some of the ideas presented here... will have to come back to TFP when I have my own!
I developed pretty early (breasts and hair), but didn't get my period until I was 13. My mom had always been open about our bodies, periods, etc, so I knew what to do, but my parents were on vacation when it happened and I had to tell her over the phone. She was more emotional about it than I was!!
Even though my mom has been a little ashamed of talking about sex, all other parts of the body were open to discussion and I've always been comfortable talking to her about that. I'm glad for that, though hell, she still barges into the bathroom when I'm visiting home and getting out of the shower!
I think if you can cultivate a level of trust and comfortability with your daughter, where she feels okay asking direct questions, that's good.