A red letter day has come to mean an important day when some noteworthy event or celebration is taking place. Originally the phrase came from the habit of ecclesiastics of making special saints' day in red ink in their calendars. This custom started very early on in the days of hand-written manuscripts.
There are 29 red letter days in the Anglican book of Common Prayer. There are also "black letter days" in the ecclesiastical calendar, but these are not days of sadness and repentance as one might imagine from their name. They signify days of lesser feasts and obligations.
It is a little known fact that a devout Anglican could ask for his children to be excused from school on red letter days to attend religious services.