The Jews have 7 days in a week which ends on Saturday: Sunday is the first day of the week. Consequently, the followers of Jesus Christ gather at the temple for prayers at the first day of the week to fellowship because our Lord Jesus Christ resurrected on the first day of the week, Jn 20:1,Luke 24:1,Mark 16:9 Historical Fact is that by the early first century, around 34 AD Christians had started gathering at the temple at the first day of the week for fellowship. This was before the time or formation Catholic Church. (Not a Roman Catholic Church), Read up about the day of the week in which the Christian gather for worship in Jn 20:19, Acts 20:7,1 Cor.16:2. I see you are combining ignorance with lies. Sunday is the first day of the week, the weekend starts on Friday and end on Saturday and the new week commences on Sunday morning. At least, you could have asked google what day is the first day of the week. And why Christians worship on Sunday which is the first day of the week against Saturday or Sabbath day which the Jews worship . Most of you who think that Monday is the first day of the week are ignorants of the historical fact.