If the calendar week in Zimbra starts e.g. with a Sunday but the desired day should be a Monday, then it can be changed in the calendar settings: Settings → Calendar → General → Week start

Previously accepted calendars are always hidden via the leading Zimbra mail system.

Log in to the Zimbra web client at https://mail.desy.de.
In the Calendar tab, the calendar to be hidden is deleted by right-clicking on Calendar.

The possibility of free / busy visibility can be adjusted in the calendar settings.

Go to: Preferences → Calendar → Permissions → Free/Busy

The standard setting here is "Allow both internal and external users to see my free / busy information".
However, this can be adapted to your own needs.

Furthermore, you can use "Invites" to regulate who is authorized to invite you to appointments.

The synchronization of Zimbra with other, external calendars, e.g. the Google calendar is not possible, so that the entries are shown in both calendars at the same time.

Shared calendars can be searched and integrated as follows:

1. Click on the wheel to the right of the calendar

2. Click on Find Shares

3. Search for the calendar in the upper search field

Shared calendars are deleted as follows:

Right click on the calendar and then delete

Error Message: "... could not be updated"

This is a bug for which there is the following workaround:
Just click forward a week and then back again.
The calendar will then update and display the Free / Busy information.

Is the display still empty?
Then the authorizations for the free / busy information should be checked and, if necessary, changed:

1. Log in to https://mail.desy.de with your DESY data.
2. Go to the "Settings" tab.
3. Go to "Calendar" in the navigation bar on the left.
4. Then scroll down until you come to the "Permissions" block.
5. Here you can select something like "Anyone can see my free / busy" information "or" Nobody can see my free / busy information view ". It is also possible to define a specific group of people who may view this information.

If embedded calendars can no longer be viewed or edited despite having the reading or writing rights, it is recommended to remove these calendars and re-integrate them again.

Please find the documentation how to proceed in this case:

General Information (click)

Outlook (click)

If calendars, existing in Zimbra, are not or no longer available in Outlook, please proceed as follows:

1. Open Outlook.
2. Select the "Zimbra" tab in the menu bar.
3. Select "Share Status".
4. Activate the desired calendars by ticking the control box and confirm with OK 
5. Close Outlook and wait for synchronization after restarting.

There is a possibility to set up a Zimbra group calendar, which can be read by external employees without a DESY account.

For this purpose, the web office would have to set up an extra website for such purposes.
Example: https://www.desy.de/room/occupancy/@@siteview

Access:
Access with DESY account is possible.
Access with DESY-IP is possible if the website on which this calendar was integrated is only accessible with DESY-IP.
Access for external parties is possible by setting up a functional account with reading access and sharing the corresponding account data. 

Visibility:
Visibility exists for everyone.

Group calendars are made available for functional accounts.
Since functional accounts do not have e-mail and calendar functionality by default, these must first be made available by the mail masters.