In the first century, when the Christian church was being born through the coming of JESUS, that was the custom, because we read in the book of Acts of Paul going in on the Sabbath day to the synagogues and joining with the Jews to preach CHRIST. And the reason Christians switched and began to worship on the first day of the week is because JESUS was raised from the dead on the first day of the week. And that came to be called the LORD’s Day.
And we read examples of the gathering on the LORD’s Day, or the first day, in Acts 20:7 - they gathered “on the first day of the week.”
And in 1 Corinthians 16:2 they were to come together and give their offerings on the first day of the week. So, that is why we gather on Sunday. JESUS rose from the dead, created a new people for Himself, and inaugurated a new humanity, a new creation with a new day of worship.
So let me sum it up like this: The reasons Christians go to church on Sunday is because we have been rescued from our sins, united with a risen, living CHRIST and with each other through faith in Jesus. And because of that union with JESUS and with each other, the Bible, GOD’s word, calls us to regular, weekly expressions of our corporate joy and thankfulness before GOD in worship — not just isolated Christian individuals scattered around but corporate gatherings praying and singing and hearing GOD’s Word on Sunday and celebrating the ordinances of JESUS CHRIST.
Today is the LORD’s Day. We gather as JESUS’ followers to worship Him in spirit and in truth. -John 4:24