The Church celebrates saints’ feast days throughout the liturgical year, and the date generally marks the day the saint died and entered into eternity - their birth into heaven, or "heaven birthday."
Sometimes, though, a variety of factors call for a different date to be assigned. This could happen when a saint's date of death will often fall during Lent. Or when another particularly popular saint is already on the calendar on that date.
The former is the case with Thomas Aquinas. He died in March (the 7th), which is often in the midst of Lent, so the Church celebrates him on the day of his birth in January (the 28th).
The latter happened with St. Therese. She died on September 30, but that is the feast of St. Jerome, so the Church celebrates Therese on the next day, October 1.
Earthly milestones
Then, sometimes, the Church chooses a feast day that highlights an aspect of the saint's life on earth, rather than their birth into eternity.
The feast of St. John Paul II, celebrated each October 22, is the anniversary of his election as pope (his death date is April 2, another Lenten collision).
A number of recently canonized popes, in fact, have gotten unique feasts:
John XXIII's feast day was placed on the date he opened the Second Vatican Council (October 11)
Paul VI's was placed on the date of his priestly ordination (May 29), though originally it was celebrated on his birthday (September 26).
The feast of St. John Henry Newman is celebrated on the date he was received into the Catholic Church.
The feast day of St. Rafael Guizar y Valencia is celebrated on October 24, because there was already a local custom of visiting his grave on October 24, which in the older liturgical calendar, was the feast of the Archangel Rafael, and thus the saint’s name-day.
The feast of St. Martha, on July 29, recently got expanded and is now a perfect day for siblings' day. In 2021, Pope Francis added Martha's brother and sister, Lazarus and Mary, to her feast day.
St. Theresa of Avila's feast day is unique. On the day after her death, or maybe in the wee hours of the day of her death, the world shifted from the Julian to the Gregorian calendar. October 4, 1582, went in a single night to October 15, 1582, and 10 days suddenly disappeared. St. Teresa of Avila, died on the night of October 4 and was buried some hours later on what had become October 15 — now her feast day.
Weddings celebrated forever
The feast of Sts. Zelie and Louis Martin is celebrated on the date of their wedding anniversary.
The wedding anniversary is also the feast day for the first couple to be beatified together, Luigi Beltrame Quattrocchi and Maria Corsini. The Church celebrates them on November 25.
If the parents of John Paul II, Emilia and Karol Sr, are one day beatified or canonized, and their wedding anniversary is chosen, we'll celebrate them on February 10.
And earth birthdays too
As well, there are three earthly birthdays on the Church calendar: Jesus’ birth (December 25); Mary’s birth (September 8); and John the Baptist’s birth (June 24).
