Genesis 2:2-3
The Seventh Day is Holy and Blessed by God
2 By the seventh day God had finished the work he had been doing; so on the seventh day he rested from all his work. 3 And God blessed the seventh day and made it holy, because on it he rested from all the work of creating that he had done.
When God finished His creation work that He had been doing He rested...And then He blesses the seventh day and made it the Holy Day of the week...The Sabbath Day, a Sunday, a day for us to think about God, study about Him, and read His Books...