May fifth is also called Tango no Sekku (Boy's day) or Shobu no Sekku (Iris Festival) This day became a national holiday, and was renamed Children's Day in 1948. It is customary for parents to display Koinobori (carp streamers) outside the house for their sons.
Some homes put out Kabuto (ancient warrior helmet) in hope that the boys will grow strong and healthy as warriors. The family celebrates the festival with Kashiwamochi (rice cakes filled with red beans and wrapped with oak leaves) and Chimaki (rice cakes wrapped with bamboo leaves). According to the Kodansha encyclopedia, the origin of the festival was from China in 839. On May 5, Chinese people hang medical herbs from the eaves of the roof in order to repel disease.
When the custom came to Japan, people used Shobu (irises) instead since irises were believed to repel evil spirits. During twelveth century, the custom was influenced by the warrior class. Since another meaning of Shobu is victory or defeat, the practice of giving little boys kites with pictures of warriors on them were spread in Japan. In the Edo period (1600-1868), streamers with pictures of carp were presented to boys. Recently, many families live in apartments not houses. They display samll carp streamers or Kabuto at their homes because of limited space.
Relevance to Urusei Yatsura
Lum, at one point, is facinated by carp streamers and puts them outside the house on a random day. This is somewhat akin to putting up Christmas lights in July. Also, Kintaro's spaceship looks like a giant Children's Day carp streamer.