I can't get calendar.app from the iOS app store
Try to restart your device whit:


On an iPhone 7 or iPhone 7 Plus: Press and hold both the Sleep/Wake and Volume Down buttons for at least ten seconds, until you see the Apple logo.
On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.


If the problem persists, make sure you make all system updates.

Try an alternative to load a Backup of your device from iTunes to restore the calendar App.