First I want to say that Editorial Calendar is a fantastic piece of work, easily one of the nicest WP plugins I use.
One issue I have though is that all its static assets -- CSS, JS, and images -- are loaded via HTTP regardless of how WP Admin itself is loaded. We require SSL for our admin area via
FORCE_SSL_ADMIN but the plugin loads these files via plain text HTTP.
This causes scary messages in some browsers and if a writer chooses to not load the insecure assets, the calendar does not load at all (this happened to one of our writers).
Overall great work though, so I'd love to see this one issue fixed. Thanks!