Today is the memorial for St. Gertrude of Nivelles, who is popularly known today as the patron saint of cats. I was wondering what the reason for that was. In some cases, it's a matter of association of the animal with the saint in some specific legend (e.g., St. Melangell for rabbits or St. Roch for dogs or St. David for doves); in other cases, it's a matter of iconography (e.g., St. Mark for lions); there are many other possible ways it can happen(like a later association with a shrine or a church named after the saint), since these informal patronages do not indicate anything formal or substantive beyond the fact that we need to draw on our poetic imaginations in intercessory prayer as elsewhere, and therefore do. So how does St. Gertrude come to be the patron saint of cats? The answer seems to be that nobody knows for sure, but the association is relatively recent, since the first definite attribution of patronage of cats to St. Gertrude is usually thought to be a 1981 catalogue by the New York Metropolitan Museum of Art. She has a long association with mice; according to legend, water from her abbey's well will chase away rodents. So the best that can be guessed is that this leads to the association with cats, who also chase away rodents, and thus can be regarded, poetically, as St. Gertrude's Knights.
In any case, here's a fairly good Mental Floss essay from two years ago on the question.