This draft is well written and is ready from an ART perspective. I found myself wanting a few more examples (e.g., for "shareWith", which I found in draft-ietf-jmap-sharing), but I'm not an expert in JMAP. 


Nits:

Section 1.4. Definition of Principal -- while there is a reference to [I-D.ietf-jmap-sharing] before the term Principal is used, the term is not defined or otherwise explained. A more explicit pointer or a definition would be helpful here. 


Section 4. The following description is hard to parse. There can be multiple principals for a calendar, but "id" is singular in this paragraph:

   *  *shareWith*: Id[CalendarRights]|null (default: null)

      A map of Principal id to rights for principals this calendar is
      shared with.  ... The account id for the principals...


Section 5. s/overriden/overridden


Section 5.8.1. The following line is too long and was truncated when I tried to print the document:

          participants~1dG9tQGZvb2Jhci5xlLmNvbQ~1participationStatus": null


Section 5.8.1. s/This would mean remove/This would mean removing


Section 6. s/The contents/The content


Section 6.3. Does the following suggestion improve readability?

   Current:

   2.  Add a new alert to the event with an AbsoluteTrigger for the
       date-time the alert has been snoozed until.

   Perhaps:

   2.  Add a new alert to the event that has an AbsoluteTrigger
       specifying the date-time when the alert is to be sent again.


Section 9.3.2. s/only be/only by


Terminology:

  o Principal is inconsistently capitalized. 

  o Quotes are inconsistently used for object, method, and property names.  

Best regards,
Jean