Great question! Here are the New Testament references that people use to infer Sunday as the commanded day of worship:
Acts 20:7 refers to the disciples breaking bread together and hearing Paul preach ‘on the first day of the week’ but the depending on how you measure the time, this meeting most likely occurred Saturday night (for a day began at sunset, not sunrise) or possibly Sunday night. They met for a meal and good teaching, but not necessarily a worship service the way we understand it.
1 Cor 16:2 shows Paul receiving an offering ‘on the first day of the week’ but again, this does not describe a church service the way we understood it, but rather the importance of supporting fellow Christians financially.
The Bible actually does not command us to meet on Sunday morning.
Sunday Morning Church got traction with the Early Church Fathers in the second century who encouraged it to distinguish from jewish sabbath keeping and to commemorate the resurrection.
Paul wrote, in Romans 14:5, “…some think one day is more holy than another day, while others think every day is alike. You should each be fully convinced that whichever day you choose is acceptable.”
In John 4, Jesus told the woman at the well that ‘true worshippers will worship the Father in spirit and in truth.” He said this when she was essentially asking the correct place to worship? Jesus’ answer indicates that worship is not so much about place (or in this case, day) but heart attitude.
So any day or night you want to gather to worship, pray, study and receive the sacraments is a “Day of the Lord.”