Children's Day

This day comes during Golden Week (see above) and falls on May 5.  It is not actually "children's day" but "boy's day" since it is typically a day set aside to wish for healthy boys and their future success. The day is filled with fun activities and special foods like rice dumplings wrapped in bamboo leaves.  Decorations usually consist of cloth streamers in the shape of carp and dolls dressed like warriors.

Since the fifth month and especially the fifth day of that month was felt to be filled with bad luck, this celebration was developed as annual ritual purification.  Now iris leaves are placed in water for boys to bathe in since the leaves are thought to have the ability to banish evil as well as have medicinal properties.

http://www.asianinfo.org/asianinfo/japan/pro-society_celebrations.htm

Kodomo-no-hi 

Originally known as boy's day, it is now a celebration of all children. Much like Girl's Day (Hinamatsuri), dolls for boys are put on display in homes. The dolls are patterned after warriors and heroes. Carp are another symbol of Children's Day. They represent strength (known to swim up waterfalls), and success in life. An ancient practice of bathing with flag plant leaves is also observed on this day; it is said to have medicinal values. It is also important on this day to make offerings of Japanese confections: rice dumplings wrapped in bamboo leaves and rice cakes wrapped in oak leaves.

http://japanese.about.com/library/weekly/aa050297.htm