"평창에서 동계 올림픽을 개최합니다."
Translation:They host the Winter Olympics in Pyeongchang.
Addendum to Pamela909830's comment.
(1) 동사 means "verb" but you are correct 겨울 is a noun, 명사
(2) 동계 is "hanja" for 겨울. It is transliterated from Chinese where 동계 means the winter season [동: winter 계: season]
The full Hanja set is "춘하추동" (similar to the naming of cardinal points, 동서남북).
(3) 춘계, 하계, 추계 and 동계 are only used in compound words, 합성어.
In such cases, as Pam said, they act like determiner 결정자 modifying the following word. e.g.
춘계 학술대회 = Spring Symposium (Academic Conference)
하계 식물 = Summer plants
추계 축제 Autumn Festival
동계 올림픽 Winter Olympics