In 1582, the reformed Georgian calendar was put into effect by most Catholic countries. The reason for the reform was that the Julian calendar, which had been the calendar for most Western countries, contained an 11 minute error in the calculation of a year, an error that had resulted in a 10 day discrepancy in the date of the equinoxes. The vernal equinox, from which the date of Easter is calculated, should be occur on March 21st, but was actually being celebrated on March 10th. In order to correct this, ten days were dropped from the calendar, and that year, October 4th was immediately followed by October 15th.
Today is also the feast day of the great Carmelite reformer and mystic, St. Teresa of Avila. The only reason I remember anything about the reformation of the calendar (despite a vague memory of studying in school at some point), is because of and odd bit of trivia concerning St. Teresa – to wit, that she died on the night that the change went into effect, the night between October 4th and October 15th. Had the Georgian calendar been put into effect on any other day, we would be celebrating her feast day on October 5th