The early church met on the "first day of the week" which in those days was Sunday because Saturday was the Sabbath (a holiday or like a weekend). Sunday was considered the first business day or commercial day of the week in those days which is like our Monday today. Out group used to meet Friday nights, now we meet Saturday nights only out of convenience and no other reason. Lots of people have really religious reasons for meeting on certain days. Ironically "everyday" is a good day to meet. Good question !!