Hey there. Today's the Buddha's Birthday Lantern Festival in Jonggak.
If you live anywhere, ANYWHERE near Seoul, get your butt down to Jonggak (near Gwanghwamun) and check out the street festival. Build a Buddhist lantern, hang out with the peoples, take pictures, and all that cool stuff.
Seriously, the Lantern Festival is the best day of the year in Seoul, and casts Korea in about the most positive light you'lee ever see it. If you don't believe me, here's my glow-in-the-dark happy post from attending two years ago.
Even more: check out the parade in the evening, from Dongdaemun all the way down to Jogyesa Temple (basically the Westminster Abbey of Korean Buddhism), and after dark, poke your head into Tapgol Park, strung up with lanterns: one of the prettiest sights I've seen in Korea, and nice enough I'm actually bringing my tripod downtown to get better pictures of it this year.