Justin It'll specifically open gnome-calendar if it is available. Not planning on adding additional features to Budgie 10.x though, but it's entirely possible that we can expand on "Default Application" selection in our own Control Center to include stuff like calendar setting, and utilize that for Raven's Calendar widget, for Budgie 11.
Expanding on that, having evolution-data-server (EDS) integration for Calendar and upcoming Events is something I look forward to adding.