Font of calendar in Preference pane of macOS
You can also use font identifiers for this, in case they have this in their database. 

https://developer.apple.com/design/human-interface-guidelines/macos/visual-design/typography/

This suggests SF pro for macOS in recent versions.


  Since its introduction, San Francisco has gradually replaced most of
  Apple's other typefaces on their software and hardware products and
  for overall branding[13] and has replaced Helvetica Neue and Lucida
  Grande as the system typeface of macOS and iOS since OS X El Capitan
  and iOS 9.[14][3][15] Apple uses it on its website and for its product
  wordmarks, where it replaced Myriad Pro. It is also used on the
  keyboard of the 2015 MacBook and on the 2016 MacBook Pro, replacing
  VAG Rounded.[16] It is also used as Apple's corporate typeface.[17]
  
  Apple restricts the usage of the typeface by others. It is licensed to
  registered third-party developers only for the design and development
  of applications for Apple's platforms.[3]


https://en.wikipedia.org/wiki/San_Francisco_(sans-serif_typeface)#Usage