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。

《新英汉大辞典》

This is a red-letter day.

这是个值得纪念的日子。

youdao

My father said that the day I was born was a red-letter day for him.

我爸爸说我出生的那天对他来说是个值得纪念的日子。

youdao

And one day, outof the blue clear sky, I got a letter from Jenny wondering if I couldcome down to Savannah and see her, and that'swhat I'm doing here.

VOA: standard.other

It was also an unhappy red-letter day for Nathan Hines, the lock winning his 50th cap but earning an early sin-binning for a swipe at Wales full-back Lee Byrne.

BBC: Fear drives Wales team forward

Tomorrow is also a red letter day for another reason - explained here in another, somewhat less impassioned letter from Carwyn Jones, this time to the Deputy First Minister.

BBC: Preparing for the crunch ahead

In the note, which reads more like a kind Mother's Day letter, Matthew thanked his "mama" for making him the man he has become and said he couldn't ask for a better mother.

NPR: APNewsBreak: Prayers Before SC Woman Killed Family