red letter day

noun2

1. •
   A day marked in red on calendars; a church feast day.

   • The only red-letter day occurring in July, is the twenty-fifth, called St James's Day. — 1825
2. •
   A particularly significant day; a day of personal or sectarian celebration.

   • Monday was a red letter day for her. She accomplished a lot and had fun doing it.
   • We saw losses for days in a row, but Black Tuesday was the worst red letter day of them all.