Is it true that ten days were lost to the world after Teresa of Avila died?
In a manner of speaking, yes. Teresa of Avila, the reformer of the Carmelite order and cofounder with John of the Cross of the Discalced Carmelites, died on October 4, 1582. Due to the reform of the calendar by Pope Gregory XIII (r. 1572–1585), the 10 days counted from October 5 to October 14, 1582, were dropped. Thus, the day after Teresa died was not October 5 but October 15. October 15 is her feast day on the liturgical calendar.
While this bit of saintly trivia can be amusing to drop on unsuspecting fellow Catholics at the next coffee-and-doughnut social after Sunday Mass, rest assured that the “lost days” were not dropped from the calendar because of Teresa’s death. They simply happened to coincide with her death.