While Americans use the fifth of May to celebrate the victory of Mexican troops over their French oppressors, throughout Korea and much of Southeast Asia the day is used to celebrate children. For elementary school children in particular, this day is one of great importance. Many children get the day off from school, and in some cases, subsequent days off as well, although I have been unable to verify if school is officially canceled or if the parents are just really, really nice.
On the holiday itself, it is traditional for parents to buy a gift for their child or to take their child somewhere special. For example, families may go to the movies, the zoo, or to an amusement park. However, my friend and I went to the shopping capital of Seoul on Children's Day to find it bursting with people, very few of whom were children. The only logical conclusion, therefore, is that the parents took the day off to "spend with their children" and instead went on a shopping spree.
Three days after Children's Day is Parents' Day, on May 8. On Parent's Day, the tradition is to buy a flower, typically a carnation, for your mother or father. This year the day fell on a Saturday, so from Friday through Sunday, convenience stores and florists sold small, premade baskets of flowers outside their stores, and street vendors switched products to sell the same. This makes the holiday pretty difficult to miss for Seoulites.
On a related note, happy Mother's Day to the greatest mom in the world! I love you!