I'd suggest the old standby remedy of moving ~/Library/Preferences/com.apple.iCal.plist to the Desktop and relaunching Calendar. In my case, that restored the default display parameters without affecting actual calendar content, which I believe is all maintained in ~/Library/Calendars/, but just to be safe, I wouldn't trash the plist until confirming that everything is as it should be.
dkmarsh—member, FineTunedMac Co-op Board of Directors
