To make sure we are on the same page, here are the usual instructions (at the bottom).
If you are syncing with an online service: You can also try turning Calendar syncing off then on again either in Settings > Mail, Contacts, Calendars > your account, or in Settings > iCloud (if that is what you are using to sync). Best to open the Calendar app in between, to give it a chance to clear.
If none of this works , it is probably going to take a restore to fix the iPad. Or at least that's all I can think of at the moment.
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When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
