"It's a great piece of luck, ma'am," said Mrs Belfield, "that you should happen to come here, of a holiday! If my son had not been at home, I should have been ready to cry for a week: and you might come any day the year through but a Sunday, and not meet with him any more than if he had never a home to come to.""Red-letter day" is one of those expressions we use quite frequently without really thinking about its ancestry. Everyone knows that a "red letter day" is one that stands out as important, or "Memorably happy," as the AHD puts it. Behind the idiom lies an actual medieval calendar tradition.
"If Mr Belfield's home-visits are so periodical," said Cecilia, "it must be rather less, than more, difficult to meet with him."
"Why you know, ma'am," answered Mrs Belfield, "to-day is a red-letter day, so that's the reason of it."
"A red-letter day?"
"Good lack, madam, why have not you heard that my son is turned book- keeper?"
Fanny Burney. Cecilia: Or, Memoirs of an Heiress.
In the middle ages, the wealthy had expensive and often luxuriously illustrated prayer books known as books of hours. These personal prayer boks provided prayers and readings tied to the various times of days, and to particular feast days in the Catholic ecclesiastic calendar. The book of hours associated the feasts days, saint's days, and other religious days in the church calendar with specific images, and prayers. Each month of the year was represented, with a list of the important dates, and, typically, an image of a seasonal agricultural or aristocratic practice (hawking in May, for instance, or harvesting nuts in November) and an illustration showing the zodiac sign for that month, for instance Gemini in May and Scorpio in November.
The illustration was either accompanied by or incorporated into a list of dates for the particular month. This list or calendar used color-coding to indicate the really important dates from the less important dates. The major religious feast days like Easter were in gold leaf; while the lesser but still important dates were in red— hence "red letter day."