Rotation Lock

  1. Swipe up from the bottom of the screen to reveal Control Center.
    Look for the icon that shows a lock with a curved arrow around it. This is the screen orientation lock.
  2. Tap the orientation lock icon to lock the screen to its current position. Tap it again to unlock.
  3. When you're done, single click the home button again to be returned to your apps

If Rotation Lock is ON this will overrule Week Calendar's rotation settings.
The latter you can find in
Settings > General > Enable device rotation