St. Teresa of Avila's feast day has been celebrated on October 15 for many centuries and it is the only date that has ever been commemorated.
What's interesting is that St. Teresa of Avila died on October 4.
When the Catholic Church declares an individual a saint, a date on the liturgical calendar is assigned to them, providing a day each year to celebrate their life.
As a general rule, this day is almost always their date of death. Since saints are believed to already be in Heaven, the date of a saint's death would be considered their "heavenly birthday," or "entrance into Heaven."
This is why you can almost always know the date of a saint's death. Odds are likely that they died on the day the Church venerates them in the liturgy.
What's curious about St. Teresa of Avila's feast day on October 15 is that it is "technically" the day after October 4.
Switching of calendars
An article for the Catholic Herald explains how St. Teresa of Avila's death happened at a unique time in history:
On Thursday October 4, 1582 the ancient Julian calendar (organized by Julius Caesar in 45 BC)...was officially supplanted at the command of Pope Gregory XIII with the Gregorian Calendar we still use today...To help correct a calendrical slippage of several days, at midnight of October 3 to 4 the calendar skipped to Friday October 15.
It certainly was a unique time to die, as her feast could have been at the beginning of October, but was then instantly moved to the middle of October.
October 15 is still a correct date to remember her death, as the calendar changed at about the same time that she died.
