Do you mean that you can add the calendar just like you can add the trackers?
If that is possible, that would be a clean solution. It may be a lot of work though 
so feel free to say no, haha
Edit: if any sensor that the user specifies, could trigger a certain scene, just like the trackers can trigger the away scene, users can pick any date/event sensor they like. (I am sorry, my ADHD brain is currently working overtime)