"Sabbath - The seventh day of the week among the Hebrews, the day being counted from sunset to sunset, that is, from Friday evening to Saturday evening"
As for Sunday it further says- "Sunday - Sunday (Day of the Sun), as the name of the first day of the week, is derived from Egyptian astrology"
So why keep Sunday?
What does GOD say?
...turn thee yet again, and thou shalt see greater abominations than these. And he brought me into the inner court of the LORD'S house, and, behold, at the door of the temple of the LORD, between the porch and the altar, were about five and twenty men, with their backs toward the temple of the LORD, and their faces toward the east; and they worshipped the sun toward the east. Ezekiel 8:15,16