I didn't know about the history of shichi-go-san either, but I became interested after I started to write this series. After asking a lot of people and doing some research myself to confirm what I was being told, I learned about this celebrations' beginnings in the Edo Period. I suspect that the history of these customs goes back even further and only became formalized during the Edo Period, but I'm not sure.

I would also guess that Girl's and Boy's Day probably started during the Edo Period as well, but I don't know for sure. I'll look into it. Buying a 'kabuto' for boys and a 'hina' doll for girls seems to be pretty traditional. I don't think everyone does it these days, but my father-in-law was pretty adamant about buying these things for my children. I was surprised to see what a big difference there is in price between well-crafted 'kabutos' and 'hina' dolls and their cheaper imitations that many toy stores sell.