Sol Invictus (“Unconquered Sun”) was the official sun god of the later Roman Empire and a patron of soldiers. In 274 the Roman emperor Aurelian made it an official cult alongside the traditional Roman cults. Statuettes of Sol Invictus, carried by the standard-bearers, appear in three places in reliefs on the Arch of Constantine, a triumphal arch in Rome. Emperor Constantine’s official coinage continues to bear images of Sol until 325.

In early days Christians were first, and horribly, targeted for persecution. In year 312 Roman Emperor Constantine got converted to Christianity. He immediately declared that Christians and pagans should be allowed to worship freely, and restored property confiscated during persecutions and other lost privileges to the Christians. He decreed (March 7, 321) dies Solis—day of the sun, “Sunday”—as the Roman day of rest: 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.

Constantine seems to have made this change himself and not through the papacy, since the papacy had not really come in to being at that time. The papacy grew gradually out of the office of Bishop and for many years this was centered in Rome. In any case, in doing this, Constantine did not change the Sabbath; he merely made Sunday the official day of rest for the Roman Empire. His motivation was probably not born out of hatred for the Jews but out of a desire to adopt what the Christians had practiced for nearly two and a half centuries.

Reference:

http://www.mapsofworld.com/on-this-day/march-7-1965-civil-rights-activists-are-beaten-by-police-in-the-bloody-sunday-march

http://en.wikipedia.org/wiki/Sol_Invictus#Sol_Invictus_and_Christianity_and_Judaism

http://www.gotquestions.org/Constantine-Sabbath.html

http://www.bbc.co.uk/history/ancient/romans/christianityromanempire_article_01.shtml