The Gutenberg Bible is having a red-letter day: tradition holds that Johannes Gutenberg printed the first copy of the Latin Vulgate Bible on February 23, 1455, in Mainz, Germany.
The Gutenberg Bible was most famous for being the first book in Western Europe to have been printed on a printing press with metal, movable type. It was a new technology, and Gutenberg experimented with it during the typesetting and printing of his Bible. One experiment he undertook was two-color printing.
It was a common thing in hand-lettered books and manuscripts for important texts (and particularly texts that introduced a new chapter) to be lettered in red. This is called rubrication, from the Latin ruber, meaning "red," and the headlines themselves came to be known as rubrics. Gutenberg attempted to automate rubrication, but the process (which required a page be passed through the press twice) was too time-consuming to do for an entire Bible. There are only a handful of pages in the surviving copies of the Gutenberg Bible that are mechanically rubricated. This practice wasn't restricted to Bibles, though: important feast days in the Christian liturgical calendar were often rubricated so they were easier to spot. These important feast days came be to known as red-letter days, and we use the adjective red-letter now to refer to something of special significance.