From now on, you can add Wunderlist to the myriad services available for integration in Sunrise Calendar. Any tasks that have a date associated with them will appear alongside any other all-day events in your calendar. This is in line with Sunrise's philosophy that they "always pair what you need to do with a time when you’ll be doing it."
Once you have connected Wunderlist, it will work just as described. If you have completed a task, it will still be there, but with a checkmark. Here's an example from my calendar, where I had a task for paying my credit card bill.
See the checkmark? Pretty satisfying. Of course, on days in which I have a lot more tasks, I can see things getting cluttered. I hope Sunrise considers an option to remove completed items or otherwise bunch them together in some way.
In addition to the Android app, this new feature applies to the iOS and desktop versions as well. It's good to see development to continue putting out a solid product under the new leadership of Microsoft.