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Careful - One could also ask, "When did God allow for days of the week to be named after pagan gods?"

God rested on the "seventh" day. The "Sabbath" was not established until the Exodus. Moreover, Christ very publicly neutered many of the Sabbath regulations that the Pharisees burdened the people with, so the issue of "what day is the Sabbath" is only relevant if one is clinging to the Mosaic Law.

The earliest known references to "Sunday" worship go back to around 50 A.D., when Christians were being persecuted in the Holy Land and it was expedient to worship on a day when there would be less conflict with Sabbath-worshiping Jewish folk.

Now, the New Testament never says that Christ was Resurrected on "Sunday", but on "the first day of the week", which IS Sunday. "The first day of the week" is also interpreted as the first day of the "New Jerusalem; the new Creation". The Catholic Catechism refers to the Resurrection as a kind of "eighth day" of Creation.

It's also important to point out that when Hadrian sacked Jerusalem in 135 A.D., he outlawed Sabbath worship. Thus, Jewish Christians in Jerusalem began worshipping on Sundays en masse.

Justin Martyr, 150 A.D.

"On Sunday, we meet to celebrate the Lord’s supper and read the Gospels and Sacred Scripture, the first day on which God changed darkness, and made the world, and on which Christ rose from the dead."

Constantine, 321 A.D., the "Sunday Decree"

"All judges and city people and the craftsmen shall rest upon the venerable Day of the Sun. Country people, however, may freely attend to the cultivation of the fields, because it frequently happens that no other days are better adapted for planting the grain in the furrows or the vines in trenches. So that the advantage given by heavenly providence may not for the occasion of a short time perish."

Catechism of the Catholic Church

The Catechism of the Catholic Church teaches that the ten commandments are "fundamentally immutable" (no. 2072). However, the Church considers the Sabbath to have two aspects: an essential part to worship the Lord on one day per week and a ceremonial part as to the exact day.

(http://www.catholic.com/thisrock/1999/9902fea1.asp)