At the crack of dawn:
Very early in the morning, especially at the time when the sun first appears.
“At the crack of dawn” 的意思是 “一天开始的时候”，也被用来表示 “天一亮、一大早就发生了某件事情”。名词 “dawn” 指 “日出的时候，黎明”。
例句
I’m so tired today. I had to get up at the crack of dawn to avoid too many people on the train.
我今天累坏了。为了避免和太多人一起挤火车，我不得不天一亮就起床。
We left at the crack of dawn so we could get to my grandma’s house on time for breakfast.
我们一大早就出发了，以便能准时到奶奶家吃早饭。
We start work early, so we have to get up at the crack of dawn.
我们很早就开工，所以必须天亮时就起床。
