I can't find my calendar app. How do I get it back on my phone?

The calendar app is a native iPhone app that cannot be deleted. So you can do a spotlight search for it (swipe the home screen right-to-left and type "Calendar"). Or do Settings > General > Reset > Reset Home Screen Layout.

Do you have some folders created? If so, check in the folders for Calendar. If you find it, drag it out of the folder.

Otherwise, Calendar should appear on some screen to the right of the home screen. Hold it down until it wiggles, then drag it back to the home screen.