Why Was Aquinas's Feast Day Moved?
Why did St. Thomas Aquinas's feast day get moved?
St. Thomas Aquinas's traditional feast day was March 7, the day he died. However, 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day to January 28, the day his remains were placed in their final resting place, so that his feast day would not be superseded by Lent.
When a saint is of particular importance to the universal Church, as is Aquinas, the Church may assign a date that doesn't conflict with major feasts so as to ensure the saint can be honored throughout the Church. (This is, by the way, also why St. John Paul II was assigned a feast day in October. The day he died, April 2, always falls during Lent, Holy Week, or Easter.)