Open the App Store > search Calendar App > find the Apple Calendar app and install it. Viola!
If that doesn't work there is another way. Text yourself a date such as March 1.
Tap that date when you receive the text and you should be prompted to open it in the calendar. Do so. Since your calendar has presumably been removed it should ask to take you to the App Store. Do this then install the calendar app. It's that simple.
|Apple Calendar App in the App Store|