A slightly late 추석 잘 보내세요! to you.
Wikipedia states that 추석 means 'Autumn eve'. The usual word for autumn is 가을, but in the biggest K>E dictionary I have, I stumbled across 추계. Is that traditional or literary? Also, I can't find anything about 석 at all.
In general, the syllables of Korean words necessarily have a meaning of their own, or are some words un-decomposable?