The use of the Sunday as the first day of the week is the typical Sunday
School answer, it is 'anatomically' correct on the surface.  However I
read somewhere that it was also a 'convenient' coicidence with the
Greco/Roman week and hence retained.

Non-evidence is the lack of 'conflict' with the Roman week.  Otherwise,
there would have been some comment in history about the change-over when
Christianity gained secular ascendance during Constantine's reign.

i.e., if Christ had risen during the middle of the Roman week (say their
equivalent of "Wednesday" there would have been some outcry about the
topic in Pagan writings of the time about having to change the
begininning of the week.