If you use Google/Gmail ..
http://windows.microsoft.com/en-us/windows-8/use-google-windows-8-rt
“To sync your calendar”
Though you can’t sync your Google calendar with the Calendar app, you can see your Google calendar events by moving them to Outlook.com. For more info on how to do that, see How to see your Google events in the Calendar app.
This is a horrible solution. It is a shame that Microsoft can’t make things work, and at least Google/Microsoft get along enough to interop. Annoying.
Steve Novoselac's Blog 