This is the calendar app your Mac should have shipped with. Fantastical integrates with Reminders, it's on all your other Apple devices and it's so easy to use. You can also dictate appointments straight into it.
You may be wondering why you need this when there's already a calendar on your device, but Fantastical offers so much more to those who really want to be productive.
For me, it's all about the integration with Reminders, as it makes such a difference having your timed to-do list entries appear next to your appointments. Fantastical also integrates with the built-in calendar app and takes full advantage of iCloud.