The Jewish prayer of Tachanun is very important to say, but there are many days throughout the year where it is omitted. This app will display the English and Hebrew date as well as whether or not you say Tachanun and why. If the background is green, you say Tachanun and if the background is red you don't.
Please contact me if you experience any bugs or find any inconsistency with halacha and the information in the app.
I use the guidelines set out by the OU for days where tachanun is omitted: http://goo.gl/SQFHXy