Standard (ISO 8601) week number in Calendar.app?
Calendar System

System Preferences in Yosemite 10.10.5 (and all later OS X releases, through El Capitan 10.11.3)  lets you change the calendar system used across apps. The default for a United States install is Gregorian. To get behavior compliant with the ISO 8601 standard, change to ISO 8601. 

Choose System Preferences &gt; Language &amp; Region &gt; Calendar (popup menu) &gt; ISO 8601.



Below are screen shots of Apple’s Calendar.app, before and after switching between Gregorian calendar and ISO 8601 calendar. I added the "Gregorian"/"ISO" labels. 

Note the week numbers shown on the left. Weeks 52 &amp; 1 changed to weeks 52 &amp; 53. In a standard week, January 1st of 2016 is in Week 53. In ISO 8601 notation for weeks, that would be the week 2015-W53, and January 1st 2016 would be 2015-W53-5 with the weekday number appended. 

For more info, see an entire Wikipedia article dedicated to ISO week date.





CAVEAT I do not know the ramifications of changing your Mac calendar system from Gregorian to ISO 8601. I searched support.apple.com but found no documentation.