You might think of March 17 as St. Patrick’s Day, but it’s also the feast day of a lesser-known saint: Gertrude of Nivelles, patron saint of cats. At least that's what the internet says, even if the Roman Catholic Church has never made it official.

Gertrude of Nivelles was born around 626 in present-day Belgium into a well-connected noble family. But she didn’t stick to the script that most noble women were made to follow in her era: When she was 10, Gertrude reportedly refused—loudly and angrily—to be married to the son of a duke. In fact, she insisted that she would never marry at all.

When her father died, Gertrude and her mother, Itta, moved to Nivelles (south of present-day Brussels) to set up a monastery, where she became an abbess. She became known for her devotion to scholarly and charitable works, and for taking care of orphans, widows, and pilgrims. She was also visited by spiritual visions and said to know most of the Bible by heart. But her ascetic lifestyle, which included long periods without food or sleep, took a toll on her health, and she resigned as abbess in 656 at the age of 30. She died three years later, and St. Patrick himself is said to have watched over her on her deathbed.

Because of her reputation for hospitality, Gertrude was originally the patron saint of travelers and the recently dead (who were seen as making their own form of journey) as well as gardeners and the mentally ill. But as the centuries wore on, she also became associated with rodents. The link may have involved early Christian beliefs: Gertrude was known to pray for the souls of those in purgatory, and medieval artists often portrayed those souls as mice. Gertrude’s iconography—the items in a painting or statue that told illiterate people who the saint was—always included mice or rats at her feet, climbing up her robes, or climbing the crozier that symbolized her role as an abbess.

The connection between Gertrude and rodents became solidified as veneration of her spread throughout northern Europe, and little silver or gold statues of mice were left at a shrine to her in Cologne as late as 1822. By then, she had become the saint one asked to intercede in the case of a rodent infestation; it was said that the water from her abbey’s well would chase away rats and mice.

In the past few decades, faithful Catholics (and cat lovers) have made the leap from associating Gertrude with warding off mice to associating her with cats. The idea seems to have started in the 1980s, more than 1300 years after she lived. Some sources say the first publication to link Gertrude and cats was a 1981 catalog, Metropolitan Cats, put out by the Metropolitan Museum of Art in New York. Since then, the idea that Gertrude is the patron of cats—and cat owners—has spread. As saint expert Thomas J. Craughwell explains it, “St. Gertrude … is invoked against mice and rats, which has led cat lovers to assume that Gertrude was a cat person, and so the ideal patron of their favorite pet.” There are now many icons and paintings of her with a cat.

Although the Vatican can make a saint’s patronage official, it has never done so with Saint Gertrude and cats. But most patron saints have been assigned their duties by popular tradition rather than by official recognition. So, if you want to get a medal of St. Gertrude to hang around your cat’s neck, go right ahead.