weekNumberCalculation
The method for calculating week numbers that are displayed with the weekNumbers setting.
"local" (default), "ISO", or a function
Specifying "local" causes the locale-specific calculation to be used, as determined by the calendar’s locale setting. This is the default.
Specifiying "ISO" results in ISO8601 week numbers. Specifying "ISO" changes the default value of firstDay to 1 (Monday).
You may also specify a function, which must accept a single Date and return an integer week number.