Hi Dawn, I'm Joel, hopefully I can assist you today. Sorry to hear you're having issues with your iCal account
When did this issue first start to occur for you?
Thanks. Have you installed any Mac OS updates recently? If not you can launch to App Store and check for any available updated and install them
Thanks. Have you tried removing the account from your Mac? You can do so following these instructions:
Does the calendar sync successfully from your iPhone or it has similar issues?
Thanks Dawn. The error does suggest an issue with the iCloud CalDAV servers. I Believe the same servers are used for either icloud.com me.com and also mac.com all tied to your Apple ID. Now I've checked the status of Apple services and they appear to fine, however, I have discovered that you may be experiencing a known issue occur to certain iCloud iCal users. Please see this Apple support discussion here: https://discussions.apple.com/thread/7250132?start=45&tstart=0
Unfortunately, this means you will need to contact Apple directly and log your issue with them as they will be best to advise next steps or fix anything they need to on their side. Sorry again about this and I wish there was something I could do to get it up and running again for you, but this is an issue you will need to log with Apple
Let me know if I can help with anything else. Also, please feel free to rate my service today, thanks