Ample evidence from history shows that the celebration of Sunday originated from pagan practices of sun worship. In March of 321 A.D., the Roman Emperor Constantine, who was at first a sun-worshiper and later a Christian convert, issued the first decree declaring Sunday to be a legal day of rest. In 336 A.D., the Roman Catholic church officially changed the observance of Sabbath to Sunday for political and economic expediency. Since then, the original Sabbath gradually gave way to Sunday observance and the practice remains to this day.
The Biblical Sabbath, however, is and has always been on the seventh day of the week, or Saturday. We must look to the Bible as our authority and keep the seventh-day Sabbath ordained by God.
extracted from True Jesus Church: Our Basic Beliefs - an Introductory Brochure