Fantastical is a great Calendar App for Mac OS X. Airmail offers integration with Fantastical. Please note that Fantastical integration will be activated only when the App is installed.
How to enable the Fantastical Integration?
Please go to Airmail Preferences > Services and enable the Fantastical integration in order to use it.
Using Fantastical with Airmail
1. Click on an email or a date that you would like to send to Fantastical
2. Dates are auto detected and highlighted in Airmail.
3. Just click on the date link and a dropdown will appear as shown below.
4. Click on Create Fantastical 2 Event
5. This will open Fantastical App with a new calendar entry.
6. Edit the entry and finalize by clicking "Add Entry".
In addition to the above process, you can also use the right click on a message in the message list or use the "z" shortcut key for the share menu without going to the message preview.