St. Teresa of Avila – mystic, writer, reformer, and the founder of the Discalced Carmelites – was born more than 500 years ago, on March 28, 1515. Her feast day is celebrated Oct. 15, the anniversary of her death. St. Teresa was honored in 1970, when she became one of the first women to be named a Doctor of the Church.

In 1577 St. Teresa’s guide for spiritual development, The Interior Castle (in Spanish, El Castillo Interior), was published. In the book, she envisioned the soul as a crystal globe in the shape of a castle containing seven mansions, each representing one stage in the journey of faith. The final stage, the seventh castle, is union with God.

Teresa’s personal life was not an easy one. She fell ill with malaria, then suffered a seizure which left her incapacitated for four days. When she awoke, she found that those surrounding her were so certain she was dead that they had already dug a grave for her beside the house. What followed were three years of paralysis, then a lifetime of continued illness which made it difficult for her to pray.

Because of the maladies which befell her, St. Teresa of Avila is called the patron of headache-sufferers. Because her autobiographical and spiritual writings have led so many to greater sanctity, she has been named patron of Spanish Catholic writers.

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One interesting aside on the life of this great saint: St. Teresa of Avila died in 1582, during one of the most unusual seasons in history, the change from the Julian Calendar to the Gregorian Calendar which is in use today.

Just a few years earlier, between 1578 and 1580, the Vatican – seeking to advance scientific inquiry in order to grow in knowledge of God – had constructed an observatory, named the Tower of the Winds, inside the walls of Vatican City. In the tower's upper room, the interior walls are covered in dramatic frescoes depicting “weather scenes” from the Scriptures: the Shipwreck of St. Paul in Malta on the west wall and Jesus calms a Storm and heals the Gerasene Demoniac on the south wall, both by Nicolò Circignani; and the Allegories of the Seasons on the ceiling, by Matteino da Siena or Pomarancio.

Once the Tower was completed, Vatican astronomers reported that on the equinox, sunlight shining through a pinpoint-sized hole in the wall did not reach a medallion on the floor, as expected; and they realized that the Julian Calendar which was in use at the time was wrong by about two hours every year, or three days every four centuries. The result was that in 1582, Pope Gregory XIII issued a papal bull adjusting the calendar. 

One notable change implemented in the new Gregorian Calendar was to adopt a system of “leap years.” While today we take for granted the inclusion of an extra day at the end of February every four years, most people probably don't realize the other parts of the formula which ensure that Easter always falls on the correct date. According to Wikipedia:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.

Pope Gregory brought the Gregorian Calendar into alignment with history by implementing another notable change: Ten days were simply “skipped” – so that people throughout the world went to bed on Oct. 4 and awoke on Oct. 15. The days between Oct. 4 and Oct. 14, 1582 never actually happened at all! It was during this mysterious long night – sometime on Oct. 4 or Oct. 15 – that St. Teresa of Avila died.