Who changed the day, and by whose authority?

The ancient Hebrews kept the Sabbath day, and since the dispersion of the ten tribes, Judah (The Jews) who returned from Babylon have carefully kept the Saturday Sabbath for many millennia. In Torah Yahweh stated 'The evening and the morning were the first day' (Gen 1.) From this scripture we understand that Yahweh considers a day's beginning to be from the previous evening at sundown. Therefore, it is reasonable to see that the Sabbath runs from local sundown on Friday night until sundown on Saturday.
It is odd that Christian believers whose New Testament (Covenant) is based on Torah (Five books of Moses: Genesis, Exodus, Leviticus, Numbers, and Deuteronomy) which Yahshua and His Apostles drew reference from continually, think that Yahweh has changed His mind about which is the Sabbath Day, or that He has decided that that Commandment doesn't matter any more! The Old Testament (Covenant) is the foundation so to speak of their belief system and their scriptures, and the Commandment about the Sabbath such a fundamental 'pillar' of their faith.
Sabbath is so tightly linked with Saturday in over 100 languages and with Bible dictionaries, Strong's famous concordance and many other teaching sources, it is amazing that the Gentile nations who are supposed to be 'brought nigh to the Covenants of promise' have changed such a fundamental issue. In all these languages the day of rest is always a variation of the word "Sabbath."
Since Roman times however, the English speaking peoples adopted the pagan name for the seventh day, Saturday which is a derivation of the Roman Pagan god Saturn. The Seventh Day Adventists and a few other Sabbatarian denominations continue to keep Saturday Sabbath, but for the most part English speaking peoples and peoples of the globe mostly consider the Saturday Sabbath null and void. It is difficult to see why Christian peoples in our world have not carried on the Sabbath Commandment considering the repeated admonition in Leviticus 23 that these Sabbaths were to be observed forever.

Why and when the weekly Sabbath was moved

After Yahshua's death (circa 30 CE) and resurrection the Christian (based then on Judaism and considered a part of Judaism) faith was led by the brother of Yahshua, James. It seems from the book of Acts that from then until his death and the arrival of Paul (towards the end of the decade) on the scene of faith in Yahshua, the church was in unity. The facts recorded in Acts state that the early Church continued to visit the Temple and that they observed the Laws of Moses.

"The venerable day of the sun."

The Sabbath day continued to be kept on Saturday. When Rome adopted the Christian religion around the late 4th. Century CE, things began to change. As the Emperor Constantine was a Pagan sun-worshiper, in 321 CE he decided that Sunday was to be a day of rest throughout the Roman Empire. This is his statement to the nation: "On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country however persons engaged in agriculture may freely and lawfully continue their pursuits because it often happens that another day is not suitable for grain-sowing or vine planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost."
In the Roman Empire there were many Pagan religions, which of course was headed by the official Pagan religion of Rome itself. One strong strand of Paganism which was popular among the Roman military was Mithraism which was the worship of a Persian God called Mithra. Gods and Goddesses were worshipped in their hundreds, and even the Emperor himself was regarded as a god. These religions involved carrying out many abominable sacrifices, and only Judaism and Christianity stood out from the morass of Paganism as believers in one God. This was a challenge to Rome. The Romans worshipped on Sundays and this was a direct challenge to Christians being in the minority.

Christians were still seen as opposition by many religious Jews and Yahshua's problems with them continued in His followers. They were still persecuted by the Jews. Also, the Jewish nation were still a thorn in the side of Rome, and persecution by Rome continued. So, the Christians, in order to feel more secure, tended to separate themselves from them. One of the ways they may have done this was to begin worshipping Yahweh on Sundays.

"The New Sabbath."

The other great blow to the observation of the Saturday Sabbath came from the Church itself, or rather what it had become since Rome had become involved in the Christian faith. The Church Council of Laodicea circa 364 CE ordered that religious observances were to be conducted on Sunday, not Saturday. Sunday became the new Sabbath. They ruled: "Christians shall not Judaize and be idle on Saturday, but shall work on that day." Although many Christians did not take this 'on board' so to speak and in Wales at least Saturday Sabbath was observed as late as 1115 CE.
So, in conclusion to 'why the day was changed,' we can see that it was a mixture of interferance by rulers and religious leaders of the Century succeeding the death and resurrection of Yahshua when the change took place. It occurred so long ago that people today consider Sunday worship normal, and that Saturday Sabbath, still observed by the faithful Jews, as being 'for the Jews'only. How blind, how shortsighted, how misled people are, and how they miss the blessing placed in the Sabbath Day to be had by all.