Why did the date for celebrating St. Thomas Aquinas’s feast day get moved?
St. Thomas Aquinas’s traditional feast day was March 7, the day he died. Nevertheless, that day almost always falls during Lent. The Church moved his feast day to January 28, the day his remains were placed in their final resting place so that his feast day would not be replaced by Lent.
When a saint is of special relevance to the universal Church, as is Aquinas, the Church may assign a date that doesn’t conflict with major feasts so as to make sure the saint can be respected throughout the Church. (This is, by the way, also why St. John Paul II was appointed a feast day in October. The day he died, April 2, occasionally falls during Lent, Holy Week, or Easter).