iOS: If you're the sort of person that's ditched clunky laptops in favour of a svelte tablet lifestyle, you'll be happy to hear that Google has finally optimised their calendar app just for the iPad.
You could have used the web version of course, but it's a little nicer to use a native iOS app, and doubly nice that you don't need to rely on the iPhone version. I use Apple devices but never really bought into the iCloud ecosystem; Apple's calendar is fine, but I'm so wrapped up in Google accounts that I usually just use Gmail and Google Calendar. (Yeah, you can sync Apple's calendar with your Google account, but I just find myself going straight to Google.)
Google Calendar for iPad is available today. If you already have the iPhone version installed on your iPad, all you need to do is update the app for the new tablet-friendly experience.
Google Calendar [iTunes App Store]