Children's Day is a holiday that is celebrated in many countries around the world. Basically, it's a day celebrating... children; basically childrens' happiness and personalities. If there's a Mother's Day and a Father's Day, why not a Children's Day? It's May 5th (Mother's Day) in both Japan and Korea. In Japan, people raise carp shaped banners on a flag pole and eat mochi. There are festivals, obviously. I know that in Korea they admit in children to amusement parks free of charge. In any case, it's a national holiday in many places... (I think about 60 or so countries to date.)
Universal Children's Day (as decreed by the UN) is November 20th, but people don't really make a big deal out of it.