A wave of dread washed over fans of Sunrise Calendar when it was acquired my Microsoft in 2015, and indeed that dread was justified. Microsoft ended update support for Sunrise in late 2015, and now we know when the app will shut down for good. The big day is August 31st, so you'll have to find an alternative before then.
In Microsoft's previous posts on this, there was no firm date for the shutdown or details on what that would mean. A new post on the Sunrise blog lays it all out, and Microsoft also weirdly went back and edited the original blog posts to include the new date. Sunrise will be removed from the Play Store around the middle of the month (it's still present as of this posting). If you download it before then, you can keep using Sunrise until August 31st. On that day, the app will stop working completely.
So, that's it. Time to move on. Microsoft and the former Sunrise team would prefer you start using Outlook, which has become Microsoft's combination calendar and email hub.