Colin Dickey has a great article on Berfrois called Tempo Shifts on the Gregorian reform. Here’s an extract:
The Gregorian Reform was motivated initially by religious purposes: the slippage was moving Easter farther into summer, creating problems with the festival calendar of the Roman Catholic Church. The solution devised by the Church was first to remove Leap Days from three out of four centennial years (thus, the years 1700, 1800, and 1900 would not by Leap Years, but 2000 would), bringing the calendar closer in line to the actual solar year. Additionally, ten days were to be dropped from the Calendar to bring Easter back in line with its date in the fourth century, when it was first established by the Council of Nicea. October 5-14, 1582, the Pope decreed, would disappear.
Read the whole thing. Great subplot on Guardian versus Trader uses of time (ecclesiastical versus commercial calendars have different needs).
HT: Alan Martin.