Over a year ago, Microsoft acquired Sunrise, the company that produces the Sunrise Calendar application. Since then, Microsoft has integrated the features from the app into its Outlook mobile apps for iOS and Android; however, the two apps have continued to coexist.
That will end on August 31, as Microsoft will sunset Sunrise Calendar. In a clearly emotional and heartfelt blog post, the Sunrise Calendar team said the following:
Whether you decide to follow us to Outlook or not, we couldn’t go without first saying a massive THANK YOU. You’ve been with us all the way and Sunrise wouldn’t have been great without all of you supporting us, sending us feedback, encouraging us to fix bugs and sharing the app with your friends.
The team also stated that it will continue to work "side-by-side with the Outlook team" and that it's "a thrilling moment for us to work on an app of this scale", speaking about continuing to work on the Outlook apps for iOS and Android. Sunrise never worked on the Outlook mobile apps for Windows 10, and judging from the blog post, that won't change.
Of course, the app will continue to work, but it won't get updated after August 31, and it won't be available through the iOS App Store or the Google Play Store.
Source: Sunrise Blog