Today's letter is P.
22 February
Today’s feast places the Chair or “Cathedra” of Saint Peter at the center of our attention – or rather the specific mission Jesus entrusted to Peter. The feast dates back to the 3rd century and is to be distinguished from the day of Peter’s martyrdom celebrated on 29 June. The feast originated around the “cathedra” of Peter, the place where the Bishop of Rome resided and governed. The “cathedra”, was a fixed “seat” of the Bishop, placed in the mother church of the Diocese – hence the word “cathedral” – which was the symbol of the Bishop’s authority who, as a successor of the Apostles, is called to protect the flock and transmit the teaching of the Gospel to the Christian community…
https://www.vaticannews.va/en/liturgical-holidays/feast-of-the-chair-of-saint-peter-the-apostle.html