1. red-letter day - Noun
2. red-letter day - Phrase
a memorably happy or noteworthy day (from the custom of marking holy days in red on church calendars)
Source: WordNetred letter day
May 12 this year is a red-letter day for Bangladesh when the country ushered in a new era by launching its first "Bangabandhu-1" satellite. Source: Internet
When something special happens, it’s called a red-letter day. Source: Internet