WeddingHappy allows you to add tasks to your iPhone's Calendar app.

From the Tasks tab, tap on the task you want to add to your calendar. From the Task Details screen, tap the Calendar button (it's on the same row as the due date). WeddingHappy will ask for permission to access your calendar. Then you can create an event. Review the event and tap the [Done] button. The task is now also an event in your calendar.