The noun: the state of nothing much happening. This is the rarest member of the family by a wide margin — you may go years without meeting it. When writers do use it, they are usually treating the quiet itself as a thing worth talking about: "the uneventfulness of village life", "after weeks of uneventfulness, the news came as a shock", "there is a deep pleasure in uneventfulness". So unlike the adjective, which leans towards relief, the noun often leans towards a flat, empty, slightly weary feeling — the quiet has gone on long enough to be noticed and named. It is literary. In normal writing you would rearrange the sentence instead and say "nothing happened for weeks".

平淡無事（的狀態）；這個名詞很少用，多出現在文學性的文章裡