The document summarizes a Xamarin seminar about building cross-platform apps for Windows Phone, iOS, and Windows 8 using C#. It outlines the speaker's experience building a "Days Until Xmas" app for each platform between 2010-2012. Key points include using different timers, controls and APIs for the countdown on each platform and sharing code by copying rather than reusable libraries. It concludes with information on sharing the app and a request for feedback.