You haven't been syncing with anything?
It might be in the backup. If you have not changed anything significant on the iPad since that sync, and have not synced again since losing the data, try this. If you've added any important documents or other media since taht sync, save them off the iPad before proceeding.
First, without connecting the iPad, go into iTunes Preferences, the Devices tab, and turn off auto-syncing.
Connect your iPad to the computer, in iTunes right click on the iPad under the Devices tab and choose Restore from Backup.
Depending on exactly when the last sync was, you may or may not get you calendars back.
I recommend that you get a Google account and start syncing your calendar with it. You can also sync your contacts if you set the account up as an Exchange account on the iPad. This will help prevent these kind of mistakes, and give's an online way to get to your calendar when the iPad is not available.
Setting up Google Sync with your iOS device : Mail, Calendar, & Contacts - Google Mobile Help