I got a good information. so I wanna tell you. shhh.... this is secret.. haha!!
there are so many festival and event on May in Seoul.
1. Changdeok-gung Dalbit Gihaeng
2. Lotus Lantern Festival(연등회 연등축제)
22% of Korean are Buddhist. so Buddha's Birthday is also national holiday in Korea. there are so many festival at temple. but you don't need to go temple. just go to Jong-no street. you can see beautiful lotus lantern at that festival.
3. Green plugged Seoul 2015
this is one of rock festival. If you have time go there. lots of bands will make you relax and ecstatic. this festival open at Nanji Hangang park near from our guesthouse.