Sometimes a place gets called the “mother church of (something),” because it was the starting point of that thing or because of its influence - like the Basilica of St. Francis in Assisi (the mother church of the Franciscans) or the Ryman Auditorium in Nashville (the mother church of country music).
As Catholics, we don’t just like our churches, we like like our churches. Every parish makes a big deal about the anniversary of the dedication of the church (usually it’s the same day as the saint that the church is named for).
But it’s usually just a big deal locally, in that parish, but nowhere else. So how come the Dedication of the Lateran Basilica gets on the universal calendar (and even takes over a Sunday)?
Because the Lateran was the first basilica to be built after Christianity was de-criminalized, and it was dedicated as mother church of all the churches, both in Rome and in the world. The first intentionally built basilica is a symbol of unity and is (in a way) everybody’s home parish.
More on this tomorrow.