The biblical answer is that it was created by God through Jesus Christ, the Word, who became flesh (John 1:10). We find this information in the creation account in Genesis chapters 1 and 2. In Genesis 2:2 we read: 2 And on the seventh day God ended His work which He had done, and He rested on the seventh day from all His work which He had done. 3 Then God blessed the seventh day and sanctified it, because in it He rested from all His work which God had created and made. To sanctify means to make Holy, and what God makes Holy, man cannot rescind.
Because Jesus was the instrument through whom the creation came into being, He also called Himself The Lord of the Sabbath when He was on this earth (Matthew 12:8). He created the Sabbath for the benefit of all mankind (Mark 2:27, 28). Although the timing of the Sabbath day may have been lost at one point, the correct day had been restored to the Hebrews at the time of Moses (Exodus 16:22,23), and they were still keeping it on the correct seventh day when Jesus walked the earth. This is confirmed by the fact that Jesus, The Lord of the Sabbath, kept the same day as the Jews at that time.
Since then, the Hebrews have kept the same seven-day cycle and the same seventh-day Sabbath down to our time, so we know which day is the biblical Sabbath. Roman Europe in Christ’s time also kept the same seven-day cycle and still do so today. However, the Roman church/state changed their day of rest to Sunday, which is the first (or eighth) day of the week, although there is no biblical mandate to do so because God’s Sabbath is enshrined in the 10 commandments. The Romans used their own authority to change the day—but why?
The answer is that Emperor Constantine, a sun worshipper, needed to unite the Roman empire under one religion. He chose “christianity” as that religion and then laid down the rules which would govern that christian religion (generally ignoring the Bible—God’s inspired Word). One of the first laws he introduced was that everybody in the cities should observe the "Day of the Sun” as their religious holiday, not the “Jewish” Sabbath (which was actually God’s Sabbath created for the benefit of all). It is fairly common knowledge that Constantine took over many popular pagan customs and called them "christian" for the sake of unity, including those customs associated with the worship of the sun. I am supplying two quotes for your information. This first quote is an enactment (one of many to follow) introducing Sunday observance into the Roman brand of Christianity, which is universal today.
First Sunday Law enacted by Emperor Constantine - March, 321 A.D.
On the venerable Day of the Sun (Sol Invictus or the Unconquerable Sun) let the magistrates and people residing in cities rest, and let all workshops be closed. [Source: Codex Justinianus, lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol. 3 (5th ed.; New York: Scribner, 1902), p. 380, note 1.]
Pagan Festivals and Church Policy
The Church made a sacred day of Sunday … largely because it was the weekly festival of the sun; for it was a definite Christian policy to take over the pagan festivals endeared to the people by tradition, and to give them a Christian significance. [Source: Arthur Weigall, The Paganism in Our Christianity, p. 145. Copyright 1928 by G. P. Putnam’s Sons, New York.]
The United Church of God believes that the only authority for our beliefs is the Word of God (the Bible), which is described in Hebrews 4:11 as living, powerful and sharper than any two-edged sword. At the same time, we do not criticise others for their beliefs because in the fullness of time, in God’s long term plan, all men will one day come to the knowledge of the truth and be offered salvation at that time.