What version of UI5 are you using? Since version 1.22 the date picker uses the unified.Calendar, which uses the locale class to define the "weekData-firstDay".
The locale classes use the Unicode Common Locale Data Repository (CLDR) to define the first day, looks like there are two locales (Arabic and Persian) which define the first day as Saturday.
"weekData-firstDay":6
/resources/sap/ui/core/cldr/ar.json
/resources/sap/ui/core/cldr/fa.json
Regards,
Jason