Saint Gertrude of Nivelles was a historical figure and her memorial is celebrated on March 17.
Gertrude was born on March 17, c. 626 in Landen, Kingdom of Austrasia, which is now located in Belgium. Unlike many other 7th-century women, she refused to be married in a pre-arranged marriage. Instead along with her mother, Gertrude founded the Abbey of Nivelles, now located in Belgium’s Walloon region. She served as an abbess there until the age of 30.
One of the reasons she’s regarded as a patron saint of cats is the fact that she was always kind and welcoming to all feline creatures which resided around the Abbey of Nivelles. She offered them food and love. Moreover, she also showed great devotion to holy souls and at the time people depicted the holy souls in her paintings as mice. Since everybody knows felines’ affinity towards chasing mice, people eventually linked Gertrude to both species.
The idea that St. Gertrude of Nivelles is the patron saint of cats dates back to the 1980s. The very first recorded document where Gertrude is defined as a patron saint of cats is a Metropolitan Museum of Art catalogue from 1981. However, her veneration as a holy protector against rodents dates back to the 15th century.
Above info from Catalogical