Save the day:

反败为胜;挽救局面

例句：

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

本来打算会输的，但Sally进了个三分球，最后反败为胜。

Late in the day:

为时已晚;坐失良机

例句：

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

忙完这个又忙那个，等到我们回到镇里的时候，天色已经很晚了。

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

现在开始谈论改变政策，已有点为时过晚。

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.

当我们听到大卫去世的消息时，我们知道对他所有忠实的粉丝来说这将是黑暗的一天。