Fantastical 2 doesn't reimagine the calendar, but it reinvents itself and brings specific additions that are practical, efficient, and thoughtfully designed. It's my favorite calendar app for OS X.
Their iOS app has been a top calendar app for years. Now they bring that to the Mac. So this endorsement is pretty high praise from Federico Viticci. It's a little pricy, but if you're looking for a great Mac calendar app, this might be for you.