I feel that it's really important to mention that the copy I was reading was an updated copy - that is, Margaret's pads stuck to her underwear with adhesive, and she did not have to use a belt. I remember that the copy I read years ago was the 'classic' version, which didn't leave me as confused as I might have been, all things considered.
Also, I had misremembered a lot of things about this book. I had thought that Laura, the girl with the large breasts, was also the one with the sunburn. And that Margaret's grandparents overlap at her house. Neither of those things really matter, but it did made me think about how many books I'm carrying around with my head that aren't really true.
Anyway, the story is sort of complicated to describe. Margaret, a 12 year old girl with no fixed religion, moves to a small town in New Jersey where everyone is either Christian or Jewish. Margaret quickly makes friends with a group of girls (they call themselves the Pre-Teen Sensations, which, unless this was another rewrite, has held up remarkably well). The girls giggle and wonder over bras, boys, and periods, while Margaret carries on a personal quest to find religion. All the time she continues talking to God, and I really like the idea that you can have a relationship with God independant of any organized religion.
I never talked about my period with my friends, and bras were a giant source of embarrassment, and not pride, but I still have a fondness for this book. I didn't read Forever, another Judy Blume book, until I was much older, so this one is the one that hit me in my formative years. I'm glad that it's still out there for kids to look through, the kind of book that they might feel embarrassed about reading but still pour through anyway. I think that Margaret would have read this book, which in a twisty way is one of the best indicators of telling whether or not a story works.