Missing days

On Thursday, October 4, 1582, Mother Teresa of Jesus -- later known as St. Teresa of Avila -- died in the Monastery of the Discalced Carmelite nuns in Alba de Tormes.

The normal practice of the church would have been to make the date of her death -- or the nearest available following date -- the date of her feast. Her feast day, however, was set as October 15. (October 4 was securely occupied by St. Francis of Assisi.) 

Why?

The Gregorian calendar -- named for the pope who sponsored the reform of the old Julian calendar that over the centuries had shifted significantly out of sync with the seasons -- was implemented in Spain on October 5, 1582. Philip II of Spain decreed the change from the Julian to the Gregorian calendar, which affected much of Roman Catholic Europe, as Philip was at the time ruler over Spain and Portugal as well as much of Italy. In these territories, as well as in the Polish–Lithuanian Commonwealth (ruled by Anna Jagiellon) and in the Papal States, the new calendar was implemented on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October 1582.

As a result, October 15, which became La Santa Madre's feast.

Protestants, by the way, refused to recognize the pope's calendar until much later. In England, for example, didn't accept it until 1752, at which time there were riots as people demanded the ten missing days back.