Given the question: I know that the answer to the question "What did Catholics celebrate?" is in "Popular opinion remained firmly behind the celebration of Mary's conception. In 1439, the Council of Basel, which is not reckoned an ecumenical council, stated that belief in the immaculate conception of Mary is in accord with the Catholic faith. By the end of the 15th century the belief was widely professed and taught in many theological faculties, but such was the influence of the Dominicans, and the weight of the arguments of Thomas Aquinas (who had been canonised in 1323 and declared "Doctor Angelicus" of the Church in 1567) that the Council of Trent (154563)which might have been expected to affirm the doctrineinstead declined to take a position.". Can you tell me what it is?
The answer is:
the immaculate conception of Mary