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Are you sure you deleted it ? To make sure you have not moved it to somewhere else, do this please. Go to Settings > General > Reset and tap Reset Home Screen Layout. If the Calendar app is there somewhere, then this will restore it back.
If you can't see it, then open App Store. Search Calendar and you will see the Calendar app there (make sure you download the correct one that you have deleted). Tap the cloud button next to it to redownload Calendar app.
Please let me know if that didn't work, so we can continue troubleshooting
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