You know something falls under the "little known Catholic stuff" heading when it takes more than two clicks to find a Wikipedia article on it...
My day planner says that, in the Extraordinary Form, today is the optional memorial of Our Lady of Ransom. Because I'd never heard that title for Our Lady before, I looked it up... well, I tried to. The Catholic Encyclopedia doesn't even have an article with that title. A Wikipedia search redirects to the page on the Royal, Celestial and Military Order of Our Lady of Mercy and the Redemption of the Captives. What a mouthful!
Apparently, the title Our Lady of Ransom is used interchangeably with Our Lady of Mercy. It is likely that veneration of Our Lady under both titles stemmed from the same visions: In 1218, Our Lady appeared to St. Peter Nolasco, St. Raymond of Penafort, and King James of Aragon, requesting that an order be founded to rescue Christians held captive by the Moors. The Royal, Celestial and Military Order of Our Lady of Mercy and the
Redemption of the Captives was founded, and claimed Our Lady of Rescue as
Word of these visions spread through Cistercian Caesarius of Heisterbach's Dialogus miraculorum (1230). The first known iconography of Our Lady of Mercy appeared in 1280, depicting three Franciscans protected by Our Lady. Today, Our Lady of Ransom/Mercy is depicted with people of various ethnicities, social standings, etc. taking refuge beneath her mantle.