In planning my 2016 Korea trip I found two websites invaluable: firstly, the Cultural Heritage Administration site, which provides a wealth of information as to the various sites of interest throughout the country. I talk about this tool here.
Secondly, there is the Korean Tourism Organisation website. This is an obvious place to look, but there’s a particularly useful section: a calendar of events of interest such as the various festivals that take place all over the country.
There cannot be a self-respecting county in the nation that does not have its own festival, from the Goseong Lavender Festival to the Hampyeong Butterfly Festival via the notorious Boryeong Mud Festival. But how do you find out what festivals are taking place at the time you are proposing to visit? Easy: query the KTO website here. It’s not 100% comprehensive (for example, it doesn’t list the Jinju Tea Culture Festival that I found myself attending this year) but it’s as good as you’ll find.
There’s also a related section for tourist-friendly performances (don’t expect an in-depth listing of serious theatrical or music performances though), and a short list of other events which don’t fit into the above categories.
Filter by region, by date, and ensure that you will never, ever, have enough time to cover what you want to see.