Calendar issue on OS X Mavericks When I try to open Calendar I'm told "You can't use this version of the application 'Calendar' with this version of OS X. You have Calendar 6.0."

Any ideas why this could be happening? I just uprgaded to OS X 10.9 Mavericks from OS X 10.8 Mountain Lion today.
It would appear that the upgrade has failed to update your Calendar app. OS X 10.9 has Calendar v7.0 and if you do have 6.0 then something went wrong.

I would suspect that there is some serious problem with your existing Calendar app such as permissions.

Delete your Calendar app and either grab a copy from another Mavericks Mac or re-install 10.9.

If you go the re-install route I would be tempted to do a reformat and clean install myself, just to make sure there are no hidden gotchas since you already have one. At the least I would boot into recovery mode and run "Fix Permissions" before the re-install.