Bugs related to the Calendar app on iDevices can result in the application disappearing from the layout. While spotlight search will find the app, the app icon is no where to be found.
Generally, resetting the home screen layout will bring the Calendar app back to its normal location on your iDevice. Note that this ONLY resets the layout of your home screen, not your actual device.
To reset your home screen layout:
1. Tap on Settings
2. Tap on General
3. Tap on Reset
4. Tap on Reset Home Screen Layout
Confirm and that's it! Your home screen app icons should be back to the Apple defaults.