Isaiah 58:13 If thou turn away thy foot from the sabbath, from doing thy pleasure on my holy day; and call the sabbath a delight, the holy of the LORD, honourable; and shalt honour him, not doing thine own ways, nor finding thine own pleasure, nor speaking thine own words:
14 Then shalt thou delight thyself in the LORD; and I will cause thee to ride upon the high places of the earth, and feed thee with the heritage of Jacob thy father: for the mouth of the LORD hath spoken it.
Sunday is God's holy day. Saturday used to be God's holy day up until the cross in 33 A.D. But once Jesus rose early Sunday morning, the day was changed from Saturday to Sunday. Nowhere in the Bible does God call Monday His holy day. Nor Tuesday. Nor Wednesday. Nor Thursday. Nor Friday. And at this time—nor Saturday. Only Sunday is God's holy day. Which is precisely why Satan has assaulted it over the course of the Great Tribulation period and in the eyes of the world has turned it into a day like any other day.
But God's people are not fooled by Satan's devices. Others may pollute and misuse Sunday the Sabbath—but God's elect still observe and delight in it and, as a result, are greatly blessed by it.