Yes, Ellen White was buried on a Sabbath. According to the book "Life Sketches of Ellen G. White," her burial took place on Sabbath day, July 24, 1915, at the Oak Hill Cemetery in Battle Creek, Michigan, where she was laid to rest beside her husband, James White.
Source: "On Sabbath day, July 24, 1915, Mrs. White was laid to rest by the side of her husband, the late Elder James White, in the Oak Hill Cemetery, Battle Creek, Mich., there to await the summons of the Life-giver." (Life Sketches of Ellen G. White, p. 462)