Those are three-day-block holidays, on the sacrosanct side of Korean social life. No regular employer would dare intrude upon them. Well, this year the Lunar New Year "three-day block" ended up being Saturday-Sunday-Monday. We got a single day off (above normal). It was out the door on Friday evening, and back at the desk, as normal, on Tuesday morning! (Well, "morning" used loosely -- office hours for us officially begin at 2 PM and end at 10 PM).
The USA, has [I think] a legal mandate to give 'substitute' days off (e.g., Monday July 5th off, in lieu of Sunday July 4th). As of 2013, South-Korea has nothing like this. This is one of the many small blemishes on work-life in South Korea. Yes, it may be one of the richest nations in the world, but so often it doesn't...act like it.
Now, though, the government is proposing adopting U.S.-style 'substitute holidays':
Beginning next year , the nation is most likely to have a substitute holiday when a national holiday falls on a Sunday. [.....]
Under the bill awaiting its passage, each of the three-day Lunar New Year and Chuseok holidays would be extended to four-day holidays when Lunar New Year’s Day or Chuseok falls on a Saturday or a Sunday. For instance, when Chuseok falls on a Saturday, the nation would take the preceding Thursday off, and it would take the coming Tuesday off when it falls on a Sunday.