Today, 26 December, was the first of the three days that make up the Christmas Triduum, i.e., the three feasts that appear immediately after the Nativity of Our Lord. They were placed there for theological, rather than historical, reasons.
The 26th is the feast of St. Stephen the Protomartyr because St. Stephen was the first to willingly offer up his life for Our Lord.
The 27th is the feast of St. John the Apostle and Evangelist because St. John, “the disciple whom Jesus loved” and the one to whom Our Lord entrusted His Blessed Mother, wrote about the Incarnation in his gospel. The first 14 verses of the gospel of St. John, which make up the “last gospel” at EF Masses, testify that THE WORD WAS MADE FLESH AND DWELT AMONG US (John 1:14). This is, in a nutshell, the reason for the Christmas season.
The 28th is the feast of the Holy Innocents because these infants were the first to suffer and die for Our Lord.