The Dedication of the Basilica of St John Lateran is one of the observances in the Catholic liturgical calendar with its own special mass readings that supplant

The Dedication of the Basilica of St John Lateran is one of the observances in the Catholic liturgical calendar with its own special mass readings that supplant the normally scheduled readings for the day (most of the liturgical calendar consists of dates relative to the start of Advent which happens on the fourth Sunday before Christmas). If I remember correctly, this is one of the feasts considered important enough to supplant the normal Sunday readings if it happens on a Sunday. (Also, worth noting that it’s one of the feasts celebrated in both the Latin and Eastern churches.)