Taken from https://github.com/trevor/calendarserver/blob/master/doc/Client-Server/calendar-client.rst and put here so I can find it later…

Additional debug logging is available by setting some preferenecs keys in the com.apple.CalendarAgent domain. The logs are sent to the standard system logging facility, ASL, and may be viewed with the Console utility, or the “syslog” command line tool. The “Sender” for these log messages is either CalendarAgent or Calendar.

To enable complete protocol logging, open Terminal and run the following two commands:

defaults write -g CalLogSimpleConfiguration -array com.apple.calendar.store.log.caldav.http
notifyutil -p com.apple.calendar.foundation.notification.logConfigUpdated

The second command (notifyutil) makes CalendarAgent re-read the preferences, because normally they are only read at startup (and CalendarAgenet is a persistent process that does not exit often).

The debug logging domains are specified using a reverse-dns style hierarchy, so to enable all Calendar logging (includes logging of account discovery), use the commands:

defaults write -g CalLogSimpleConfiguration -array com.apple.calendar
notifyutil -p com.apple.calendar.foundation.notification.logConfigUpdated

To disable Calendar debug logging, run the commands:

defaults delete -g CalLogSimpleConfiguration
notifyutil -p com.apple.calendar.foundation.notification.logConfigUpdated

To select all Calendar and CalendarAgent logs from ASL, use Console to select these two Senders from the utility box in the left of the Console window, or use the following syslog command:

syslog -k Sender CalendarAgent -o -k Sender Calendar