The best calendar I've tested and used and of course is a completely integrated part of the stacks ecosystem. If you need language translations, the standard %lang(en:Date,de:Datum)% works all over. The data entry fields while not Hipwig, accept HTML.
Having a separate page to populate the event lightbox gives tremendous flexibility in customisation.
Scott also has a group of easter eggs in the stacks - hidden classes that can be used to customise the lightbox. cv_modal-header for the background colour of the banner, cv_modal-title for the banner text colour, cv_modal-close for the colour of the "X" to close the lightbox and cv_modal-body for the colour of the body of the lightbox (the event page is positioned on top of the body).
Repeating events are coming at some point.
It's a cool set of stacks!