iTunes should have created a backup before you restored. Did you recall getting a message that iTunes was going to backup the iPad before the software was restored?
You can check to see if you have a backup by launching iTunes on your computer and go to Edit>Preferences>Devices>and see if there is a backup for your iPad in there. If there is one, and the backup was dated before you restored the iPad, you can restore from that backup and that should retrieve your calendar entries.
This will tell you how to restore from a backup.