I just discoverd a web page (http://www.medievalgenealogy.org.uk/cal/medcal.shtml) where some hearty soul has calculated the dates/days of the week from 1100-1500 AD. Thus, for the book I’m writing now, I discovered that 11 December 1282 was a Friday. It was also the 3rd day before the Ides, which was a Roman way of figuring the days.
The Roman calendar was originally based on the first three phases of the moon, with days counted backwards from lunar phases. The new moon was the day of the Kalends, the moon’s first quarter was the day of the Nones, and the Ides fell on the day of the full moon. (Thus, Julius Caesar was murdered on the Ides of March, or March 15)
December 11th was the Feast day St. Damasus, who commissioned the translation of the Bible from Greek to Latin in 366 AD. It is also the Feast Day of St. Cian, a Welsh hermit from the 6th century, and ten other saints. Llywelyn ap Gruffydd was purportedly very devout, and attended mass, despite the fact that he was excommunicate, the night before he died.