Full integration, editable calendar with two-way synchronisation – more complicated than it could be: https://frightanic.com/apple-mac/thunderbird-icloud-calendar-sync/.
For the sync of contacts, the URL is: https://<serverID>-contacts.icloud.com/<user>/carddavhome/card/. See: https://support.mozilla.org/bm/questions/1384897.
Read MoreCreate and share meeting polls. With a meeting poll, an appointment can be made with several people. With a booking list, predefined appointments can be booked by other people: https://terminplaner6.dfn.de/.
Read More[German] Large collection of Outlook Tools and Add-ins, for mails, calendar, contacts, security/virus scanning, and more: outlook-stuff.com/tools.html.
Read MoreFantastical 2 is a calendar app for Mac that lets you quickly toggle multiple calendars on or off with a single click, so you can focus on what’s more important in that moment. The days of going back and forth, clicking multiple times, just to hide and show your calendars are over: https://flexibits.com/fantastical.
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