Genesis 2:1-3

¹ So the heavens and the earth and everything in them were completed. ² By the seventh day, God completed His work that He had done, and He rested on the seventh day from all His work that He had done. ³ God blessed the seventh day and declared it holy, for on it He rested from His work of creation.

God finished His creation within six days. On the seventh day, He rested. Because of this, he declared the seventh day holy, which will be known as the Holy Sabbath. The Sabbath was observed on Saturdays until approximately 364 A.D. This was due to the fact that at the time, relations between the Jews and Christians were hostile, because Jews were persecuting the Christians. Also, the Jews were immensely persecuted by the government, so Christians wanted Christianity to be a completely separate religion from Judaism, rather than a sect of it.