It was one of these beautiful crisp September days when the woods are adorned in autumnal browns and golds.

crisp 是形容词，本义指"脆的、易碎的"，引申为形容天气时表示"清新凉爽的"，特指那种干燥、凉爽而令人舒适的天气，常用来描述秋高气爽的感觉。