Fantastical 2 for iPad, by Flexibits Inc, is a great calendar app for iOS. Recently it was updated include widgets and app extensions to make it easier to use.
Now, using the Today widget, you can view your calendar and event list in Notification Center. The app also allows you to create events or reminders from selected text in extension-enabled apps, making it faster and simpler to add things to your calendar. Flexibits has also fixed some bugs and added interactive notifications.
Fantastical 2 is currently 20% off for a limited time, on sale for $7.99.