Building an encrypted calendar is at the top of our to-do list right now as we desperately need a solution to schedule and share calendar entries securely ourselves. However, first, we have to finish the new client to push it our of beta and update the Tutanota Android and iOS apps so that you can use your encrypted mailbox with very good speeds and many more features on all your devices. Once we are done with that, we will build the encrypted calendar.
Thank you very much for your continuous support.Demetris commented
An end-to-end encrypted calendar service is, in fact, available: have a look at http://www.etesync.com
It would be nice, however, to have a calendar service integrated with the tutanota email service & contacts.