Any day that is very important for some reason can be described as a red-letter day.
- The day my twins were born was a red-letter day for me and my family.
- Today is a red-letter day for our cricket team. For the first time ever, we have won the World Cup.
The days of church festivals are marked in red on calendars. In earlier times this phrase was only used to refer to a church festival or saint’s day. But nowadays any special day can be described as a red-letter day.
Note that this phrase can be written with or without the hyphen. Both red-letter day and red letter day are considered correct.