Trello have a Mac app, which I use for work and it struck me this morning that several recurring calendar events I have, which exist to remind me to review a particular board, would be much more pleasant if they contained a link that would open the board directly.
That would be easy if I used the Trello website, but I quite like the app (even though it's really just a browser pretending to be an app), so I went spelunking.
To cut a long story short, the Trello Mac app registers itself as a handler for trello:// URLs, so if you take any trello.com board URL and replace the https:// part with trello:// you can use it as a link in your calendar (or anywhere else) and it will open the board in the app.