My calendar no longer opens. Any ideas?

I'm having the exact same problem. Weird, any help would be great, thanks

Try updating your operating system. I've updated my OS and this has solved my problem, and the calendar opens.

There's a glitch with the calendar app. If you have an iPad 2+ update the operating system and that fixes it. If you have an iPad 1 you need to edit/remove any all day appointment/event from April 1st, this includes subscribed calendars like holiday calendars.