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The articles below may be able to help you with this issue.
Click on the links to see more details and screenshots.
First try quitting and relaunching the Calendar app:
Another troubleshooting step would be to turn the iCloud calendar off and back on.
The iPad User Guide has some information about the Calendar app on page 60.
iPad User Guide
Turn on iCloud, Google, Exchange, or Yahoo! calendars. Go to Settings > Mail, Contacts,
Calendars, tap an account, then turn on Calendar