May 5th has been made a national holiday, called since World War II "Children's Day" (Kodomo-no-hi). In principle, it celebrates both boys and girls, and koi-nobori may be flown for each family member. Displays of miniature weapons are still a part of the ceremony, but in general there is less emphasis on samurai virtues and history.
May 5 has other names. The fifth month is the month of the horse, so its fifth day is Tango no Sekku, the festival of the emperor’s white horse. This day is also traditionally Shobu no Sekku, the festival of the irises (in the old Japanese calendar, the day would fall closer to the summer solstice than it does in the modern calendar). The iris flower or flag has long been used in purifying rituals on this day, by farmers, townspeople, and courtiers. At least as far back as the twelfth century this was a day for gathering iris leaves and flowers for use in the bath, as an addition to food and drink, and in decorations on the roofs of houses and shops. The iris bath is still popular on this day in Japan.
During the 12th or 13th century the word "shobu" came to be associated with its homonym meaning "military spirit", and people started celebrating Shobu no sekku by decorating paper samurai helmets with irises. Thus it became a day for boys, as the third day of the third month was a special day for girls, Hina matsuri.
One military tradition that has evolved and persisted, though, is the setting up of a special display celebrating warrior values and Japanese heroes. In fact, it originated in the seventeenth century, when actual civil wars began to fade into the past. It may have begun with a custom of making helmets out of iris leaves, which evolved into the crafting of beautiful lacquered display helmets for the day, along with painted banners and displays of real weapons. By 1700 dolls representing warrior heroes, Musha ningyo, had become very popular, and they still are today. Besides miniature helmets, weapons, and suits of armor, the display might include a white horse, representing the Emperor, and perhaps a tiger.
Nowadays the dolls usually look like sweet little boys and not terrible warriors, and the modern name is Gogatsu ningyo, or "May dolls." Still, along with the carp banners, they are a reminder of Japan’s history and of the evolving and complex Japanese love of nature, ambition, artistry, and children.