From Bulbapedia, the community-driven Pokémon encyclopedia.
Kids Day (Japanese: こどものひ Children's Day) is a holiday celebrated throughout the Pokémon world. It is the equivalent of the real-world Japanese holiday Children's Day (Kodomo no Hi), as well as International Children's Day.
Like in the real world, it is a day to honor and respect children. On this day of the year, children receive special gifts and respect from parents. They are either excused from school or enjoy special events at school (such as having a day to play with Pokémon at school). Sticky rice cakes wrapped in Daimyo Oak leaves are traditionally served on this day, and banners in the shape of Magikarp are flown.
Kid's Day was represented in the anime episode The Purr-fect Hero.