Mac Calendar has stopped allowing events to be entered on Sundays from month view?
Quit iCal, and try removing the com.apple.iCal.plist file from your Mac


Open Finder
Go Menu, hold the option key and click on Library
Go to Preferences
Remove the com.apple.iCal.plist file or put .bak behind it (backup)
Logout or reboot your Mac and your problem should be fixed.