Request the ctag again on the calendar. If the ctag did not change, you still
have the latest copy.
If it did change, you must request all the etags in the entire calendar again.
Get the display name and the ctag.
This ctag works like a change id.
Every time the ctag has changed, you know something in the calendar has changed too.
calendar-multiget REPORT is used to retrieve specific calendar object resources
from within a collection, if the Request- URI is a collection, or to retrieve
a specific calendar object resource, if the Request-URI is a calendar object
resource.
REPORT performs a search for all calendar object resources that match a
specified filter. The response of this report will contain all the WebDAV
properties and calendar object resource data specified in the request.