History of the Sabbath and First Day of the Week - John N. Andrews

"Very shortly after the period when
Constantine issued his edict enjoining the general
observance of Sunday throughout the Roman
Empire, the party that had contended for the
observance of the seventh day dwindled into
insignificance. The observance of Sunday as a
public festival, during which all business, with the
exception of rural employments, was intermitted,
came to be more and more generally established
ever after this time, throughout both the Greek and
the Latin churches.
There is no evidence however that either at this,
or at a period much later, the observance was
viewed as deriving any obligation from the fourth
commandment; it seems to have been regarded as
an institution corresponding in nature with
Christmas, Good Friday, and other festivals of the
church; and as resting with them on the ground of
ecclesiastical authority and tradition."[29]
This extraordinary edict of Constantine caused
Sunday to be observed with greater solemnity than
it had formerly been. Yet we have the most
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