Throughout the liturgical calendar, there are a handful of feasts for the dedication of various "major" basilicas.
These basilicas are located in Rome and have been celebrated by the Church for many centuries.
The word "major," doesn't distinguish its size or architectural beauty, but it's historical importance.
The Catholic Encyclopedia gives a brief explanation.
Furthermore, since they are so closely tied to the pope, "they possess a papal throne and an altar at which none may say Mass except by the pope's permission."
These major basilicas are also some of the oldest churches in Rome, dating back to the 4th and 5th centuries.
There exists no other major basilicas outside Rome, though there are many "minor" basilicas scattered on nearly every continent.
