It's already mid-April, and 2 weeks later, Japanese peple have the Golden week which is 4-5 days holiday in early May.
One day in the Golden week, there is a Children's day on 5/5.
There is a Japanese tradition that we are celebrating this day with putting on the Koinobori which is a flying colorful carp banners, and also we put the Samurai's helmet insdie the house.
The reason why we put these is because the helmet and armor is recognized as symbolic icon of to save the boy and human being from any kind of accident, sich and disease. So people put it with a with in their mind.
Also there is a reason why we put the Koinobori outside the house, it's based on the old stories of China, it is said that only the strong carp who achieved to arrive the upper reaches of the biggest river can be a dragon. So people put the Koinobori with wishing about the kids to live strong as dragon.