There is no mandated day to attend a gathering of believers - period. Traditionally we attend church on Sundays because that is what is seen in the NT. The primarily Jewish church in the New Testament would have likely also attended Synagogue on the Sabbath since they were Jews but in Christ, especially for the non-Jews or "gentiles" there is no mandate or command to attend a gathering of believers (which is what church is) rather we are called to live our lives in obedience to Jesus every day of the week. If we can get this right then there will likely be gatherings throughout the week continually where we meet with believers.