I think I was about 9 or 10 when my mom gave me 2 books "Where Did I Come From?" and "What's Happening To Me?" I'm sure the 1st is obvious, and the 2nd went through the developmental stages of illustrated boys and girls in pretty good detail. No photos, but the illustrations showed enough of the stages side by side that I got an idea.
My mom was very communicative, always let me ask any question and ALWAYS answered (even if once in awhile the answer was "you're not old enough to understand that yet", like when I heard her tell a friend that someone was fired from their job for being gay, and I asked "Why did someone lose their job because they were happy?")
But body parts, periods, boobs... all were communicated about very openly, which I really felt like served me well.
I also remember stuffing my bra in 2nd grade (I had these little matching bikini underwear sets and would put cotton and socks in them and actually thought that because no one said anything, they didn't notice and I was getting away with it!!!). I don't remember my mom questioning me about it, although I may have waited until I got to school to do it, but since no one ever said anything I just stopped eventually and forgot about it. By the time I started to get real buds, I wasn't really phased about it.
My mom was excited when I got my period, but I felt it was more like a hassle. The great thing though is, I knew EXACTLY what was happening, what it was, to expect it, and therefore it didn't take me by suprise, I didn't have any shame, and I could just learn to deal with it and move on. In retrospect that feels like a great gift, especially when I know so many women who never knew a thing about it and were so scared when suddenly they started bleeding.
I think it's great some of you want to celebrate with your girls, and that's exactly what I'm going to do with dd. I'm not gonna get too into it, but I'm gonna explain how periods are one of the signs that girls and women are incredibly powerful and important, and I'll develop some kind of ritual (or read up on already existing ones) about celebrating that. The breast buds... I'll probably just point them out (and body hair as it comes) and say how cool they are, but not make a big deal beyond that.
And I'll always invite my kids to talk to me about ANYTHING having to do with their bodies, or if anyone makes them uncomfy about their bodies to tell me.