The weekly commemoration of the Resurrection and appearance of the Lord to His disciples afterwards (John. 20:19,26). Constantly it is the first day of every week.
Sunday has been kept by Christians since the apostolic era as a day of joyful worship. St. Paul and the Christians of Troas gathered on the first day of the week "to break bread" Acts 20:7. Besides the apostle on writing to the Corinthians, asked his converts to collect their alms on this day (1 Cor. 16:2).
Dictionary of church terms. — St. GEORGE'S COPTIC ORTHODOX CHURCH SPORTING - ALEXANDRIA EGYPT. Fr. Tadros Yacoub Maiaty.