Save the day:

反敗為勝;挽救局面

例句：

The team was expected to lose, but Sally made three points and saved the day.

本來打算會輸的，但Sally進了個三分球，最后反敗為勝。

Late in the day:

為時已晚;坐失良機

例句：

①What with one thing and another, it was fairly late in the day when we returned to town.

忙完這個又忙那個，等到我們回到鎮里的時候，天色已經很晚了。

②It’s a little late in the day to start talking about policy changes.

現在開始談論改變政策，已有點為時過晚。

A black day:

倒霉的一天;黑暗的一天

例句：

When we heard the news that David had died, we knew it was going to be a black day for all of his loyal fans.

當我們聽到大衛去世的消息時，我們知道對他所有忠實的粉絲來說這將是黑暗的一天。