Countdate lets you know how many days are left to a specific date. Next to the Android app, there's also a web version available.

Countdate is and will be free to download and use. As it's open source and under a free license, you can even take the code and modify it!

Android: Ready to use android app.
Web: Web version of countdate app is also available.
Free to Use: As always, this app is free to download and use!
Open Source: Since this app is MIT licensed, you can modified it!