Is it proper that sabbath obligation of OT is substituted by Sunday obligation?
Jesus became Lord by resurrection (Jn. 20:28). It was on Sunday that he resurrected and appeared to his disciples (Jn.20:1,19). Hence from Jesus' resurrection onwards the disciples began to celebrate Sunday as the Lord's Day. It is mentioned as the First Day (Jn. 20:1, 1 Cor. 16:2). On the first day of the week, the disciples gathered to break bread (Acts 20:7), and to take collections for the poor (1 Cor. 16:2). Since Jesus' resurrection is the very foundation of Christian faith (1 Cor. 15:14-17), it is proper that Christians celebrate Sunday as the Lord's Day, the day of obligation. It is to be noted that collections for charity existed in the early church (1 Cor. 16:2) and had the indirect sanction of Jesus (Cf. Jn. 13: 28-29). Further, the early christian community was a sharing community (Acts. 2:42-47) and real repentance envisaged charity works (Lk. 3: 10-14).