What is scarlet day?

Scarlet day is the term used in the University of Cambridge to designate those days on which doctors are required to wear the festal form of academic dress. It is so called because of the scarlet elements in the gowns and hoods of the festal full dress worn by doctors as opposed to the everyday black gowns. On these days, it is also permitted for members of the University to wear the academic dress of other universities from which they have obtained degrees.