What is the origin of the idiom "red-letter day" and when to use it?

The origin of the idiom "red-letter day" can be traced back to medieval manuscripts. In religious texts, particularly in the Christian tradition, important feast days and holy days were traditionally written in red ink or highlighted with red letters to signify their significance. The practice of using red ink for these special days can be traced back to as early as the 8th century. It is commonly used to describe personal milestones such as birthdays, weddings, graduations, or anniversaries, which hold special importance and are celebrated with joy and enthusiasm.