Go back and look at the practice of the early Church, the practice of the believers who were alive during the time of Christ and shortly thereafter.
In addition to the fact that the members of the early Christian Church lived close in time to the Apostles (sometimes with firsthand knowledge of Apostolic teaching), the early Church's testimony is also important because Scripture calls the Church "the pillar and foundation of truth (1 Tim 3:15)." Jesus also says that those who will not listen to the Church should be treated as pagans and tax collectors (Mat 18:17).
The question was, "Isn't Sunday attendance sort of a mandantory part of the Christian faith?"
It is not mandatory. Jesus Christ did not say, "Ye shall attend church services every Sunday in order to be saved."
It's desirable, sure, but not mandatory.