This is a red-letter day for you.
a red-letter day直译过来就是：“有红色字母的一天”，这个短语的正确意思是：“值得纪念的或喜庆的日子”。因此，当美国人说"This is a red-letter day for you."时，他/她要表达的意思就是"Today is veryimportant for you."、"It's a special day for you."。
Shirley: I'm so excited that I'm raised to management at today's conference.
Jane: Wow, this is a red-letter day for you. Congratulations!
①The boss praised me for my outstanding performance.
②I got promoted today for hard working.
③My idea is approved by the board of directors.
④Today I will sign a contract with the government on behalf of the company.
1. red-letter adj. 值得纪念的
People are well dressed on red-letter day.
This has been become internationally one celebrates a red-letter day.
This is a red-letter occasion.
2. board of directors 董事会
The Board of Directors has approved the decision unanimously.
The board of directors canvassed his design thoroughly.
He's trying to curry favor with the board of directors.
3. on behalf of 代表
The agent spoke on behalf of his principal.
Governor appointed by the queen on behalf of the exercise of authority.
You said you would have one more beer on behalf of your brother that day.