To display any info on your Magic Mirror, you have to use software to fetch it for you. There are several github projects available for free, that you can download and use for your project. They are mostly PHP based and easy to edit to fit your needs.
Magic Mirror Source code
The Magic Mirror GitHub project by Evan Cohen is a good place to start. You just set it up in a folder on a web server, edit it to your specifications and point your device’s browser to it.
The code displays the time, weather, calendar events, news headlines and a compliment.
Other API sources
Forecast.io – 1000 API calls per day included for free
Yahoo Weather API – No API key required!