If you haven't moved away from your native calendar app, now is the time.
Sunrise Calendar (free, Sunrise Atelier) is, in my opinion, the best native calendar app replacement out there. The design boasts beautiful, simple and modern design as well as a monthly view and daily view like the native iOS app. It also allows users to add Google, Facebook, and iCloud calendars with ease. Reminders can also be seen in the calendar view, if you wish. The latest update lets users add "interesting calendars," like sports schedules, moon phases, and holidays.
The app first came to the iPhone. Next, the developers brought it to iPad. Then, web. And now, to the Mac app store.
All three of my calendar apps (iPhone, iPad, and Mac) update seamlessly and are easily managable from any platform. Even though it is visually similar to the native Apple calendar, my favorite addition is the logo next to each event. When certain keywords are typed into event titles, like "dinner", "game", or "meeting", the colorful calendar logo changes to a fork and knife, a championship trophy, or speech bubbles.
Other features that you may find useful are the Evernote connection, Facebook event connection, Facebook birthday connection, and more. Try it out—but make sure to turn off notifications from your native calendar once you do!