Nature is embedded in every aspect of Japanese culture, and holidays often have flower or plant symbols. Children's Day (Kodomo no Hi) on May 5 has many symbols, the most obvious being the koinobori carp flags that families fly outside their houses for each boy inside. The carp, able to swim against the current, evokes perseverance and determination.
Irises are also a big Children's Day symbol. The leaves are used to wrap mochi cakes, and they're thrown into hot baths.
The flowers happen to bloom almost exactly on May 5 in beautiful purples and yellows, so wandering around Japan's beautiful gardens is a great way to mark the holiday.
Some other incredible iris gardens: • Meigetsuin Temple, Kamakura • Hondoji Temple, Chiba • Suigo Sawara Aquatic Botanical Garden, Chiba • Meiji Jingu inner garden, Tokyo