You can disable S Planner in system settings in Android 5.0 Lollipop, this app is not needed anymore if you have Lollipop. Follow instructions in the screenshots to disable it.

If you don't have this option in system settings, get adb command from Android developer tools, plug your phone into PC, enable USB debugging in Android settings, and run this command on your PC:

adb shell pm hide com.android.calendar

If nothing helped, you can try installing this app, but this is not recommended, as i