Are you talking about the Calendar App that comes pre installed on the Fairphone?
If so you can’t actually uninstall it, because it is a System App.
Maybe you accidentially removed it from the Home Screen?
You can put it back by clicking the “Settings Softbutton” on your Phone, then tap on Apps, hold the Calendar App and release it on a free space on your homescreen.
Ok try the following:
Go to Settings > Apps > swipe to the right twice to see “All” Apps. Can you find your Calendar App somewhere on that list?
Maybe you have “Disabled” the App, so tap “Enable” and it should work again.
If you really have removed it from your Phone (by making it a user app first with e.g. System/app_mover) you’ll have to do a software update of the OS to get it back.
Go to the Fairphone Updater App and install the latest Version.