South Korea has two official "major holidays" (연휴), one in Fall and one in Winter. At these times, everything shuts down on the day of, and the days before and after, a three-day block. When the main day falls on Monday, it means Sunday is one of the block days, which is kind of a gip. This time, though, by national law, Wednesday is off, too. Korean employers often cavalierly break these kinds of laws relating to labor, but are much more hesitant to impinge on a "major holiday".
The holiday that Koreans are currently observing is the East Asian Harvest Festival, called Chuseok [추석] in Korea and "Moon Festival" in other places, I'm told. Some Koreans call it "Korean Thanksgiving" as a two-word explanation of it to Americans. Its date changes each year in accordance with that other calendar they use.
By reputation, something like half the people in the Seoul region clear out and return to their hometowns at the major holidays, though especially the Winter one (what we call "Chinese New Year"). I myself will be visiting my friend J.A. in a city in the middle of the country, leaving Monday afternoon. We'll see how much of a delay there is on the bus.