Reinstall deleted calendar on iPad

See >>> If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support

Hmm...

That may not be possible without erasing your old iPad and reinstalling iPadOS 10.3.3.

I am afraid Apple supplies NO older calendar app to redownload from the iOS App Store.

Alao, unfortunately, totally resetting and reinstalling iOS 10.3.3 will render the functionality of this aging iPad to a very limited capability as there NO MORE older, third party apps for an over 10-years old iPad model.

You will lose all third party apps and all of their data.

I sincerely doubt that you will find an iOS 9 or 10 third party calendar app that supports your aging iPad model.

Sorry,

HA!

You’ve lucked out!

From the iOS App Store

Apple Calendar app

https://apps.apple.com/us/app/calendar/id1108185179

Still compatible with iOS 10!

Download/reinstall it!

Best of Luck to You!

You are very welcome.