An artifact at the recent 'Bushido - Way of the Samurai' exhibition at the National Gallery of Victoria really caught my eye. I've been researching it but have only been able to find information about the Boy's Day, not the armour. I would love it if anybody could provide some leads on reading that I could use to read on my own time :)
There are two theories on it's origin, but they all come from the "warrior society" of pre-modern Japan.
The practical origin: During the early May, armours and helmets would be set up inside the house. This is also the same time when flags & windsocks, what eventually became the Koinobori, would be hung outside. This was done so maintenance and cleaning could be done before the Japanese rainy season, which begins in June.
The spiritual origin: Having armor set up inside the house was said to ward off illness and bad luck. The origin of this belief is nebulous, but most likely stems from the simple idea that armor protects the wearer.
However until the Edo Period this was done more or less exclusively by the Samurai. It was during this period when the commoners began imitating the Samurai and putting up armor and weapons as decoration. At this point a "doll" was added to the armor, as a symbol for the gods to use when "appearing". Eventually the doll and the armour became considered separate things, and became the Satsuki Ningyou. This is also when the "Armor" became decorative fakes, instead of real Samurai armor. After all, the peasants don't own any.
Can you read Japanese? I'm afraid I don't know any books on the subject in English.