Historical reasons. Sunday has always been the first day of the week. Because yet another religion than the 4 mentioned before explicitly defines Saturday as th

Historical reasons. Sunday has always been the first day of the week. Because yet another religion than the 4 mentioned before explicitly defines Saturday as the last day of the week, and everybody has always gone along with that. Probably because it was always like that anyway. The recent official standardisation of Monday to be the first day of the week was a mistake.