red-letter day用来形容有重大意义、吉利的日子；喜庆日；重要纪念日；大喜日子。

【例句】
June 2nd will be a red-letter day for him, he's getting married.
六月二日是他的大喜日子，他要结婚了。

When I found out that I was admitted to University of Oxford, it was a real red-letter day.
那天我得知自己被牛津大学录取了，真是个大喜的日子。