red-letter day: (n) A memorably happy or noteworthy day. A day of significance.
The story of the red letter...
The tradition is traced back to the Medieval times when the church began marking Holy days and feasts in red ink on the calendar. Manuscripts were sent highlighting these significant events in red ink. It has since evolved and many holidays and special occasions are often marked in red on todays calendars. A red-letter day is meant to be a very special day, worthy only of the finest red ink.