Not true. The Church had been keeping the First Day of the week from Day One of the New Testament era in the book of Acts. It was a holy celebration of the LORD's holy Resurrection Day. Slander against Constantine the Great is common among people who are ignorant of the meaning of history. God worked through Constantine to establish justice and peace in the Orthodox Church. A great thing happened in 325 AD with the First Ecumenical Council of the Christian Church at Nicea in Asia Minor.
Ellen G. White is a proven false prophetess who plagiarized many of her ideas from other heterodox (heretical) sources. She suffered from a nervous mental disorder and was perhaps epileptic. In any case, whatever her condition, she is not a source of sound doctrine, and the Seventh-day Adventist sect is based on the cult of William Miller who heretically tried to predict the second coming of Jesus Christ in 1843 and 1844. The adventists still believe Christ "came" in a spiritual sense into the heavenly tabernacle in 1844. There whole theology is false therefore, because it mixes a few correct opinions with a whole lot of errors.
Sabbatarianism has been declared a Judaizing heresy by the Orthodox Catholic Church.
In Erie PA Scott R. Harrington