In 1582 Pope Gregory XII - born in Bologna with the name Ugo Boncompagni - implemented a calendar reform based on the studies of Nicolaus Copernicus.
To realign the spring equinox with March 21, ten days had to be removed. Preserving the traditions of his family of origin, the Pope allowed the people of Bologna to celebrate the patron saint and Thursday 4 October was followed by Friday 15 October.
