Why can't I enter events into my Calendar

Question

I used to be able to enter events in my Calendar. Now I can't. I don't care about synching with any other service such as g-mail, iCloud, etc. Adding events is greyed out.

Answer 1

1. Back up all data.
2. If there are any calendars on your Mac that you don't synchronize with iCloud, export them by selecting them in the calendar list and then selecting
File ▹ Export ▹ Export…
from the menu bar.
3. Quit Calendar.
4. Triple-click the line below on this page to select it:
 ~/Library/Calendars
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A Finder window should open with a folder selected. Move that folder to the Desktop.
*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.
5. Launch Calendar. Your iCloud calendars, if any, will initially be gone, but they should resynchronize with iCloud. If they don't, uncheck the box marked Calendars & Reminders in the iCloud preference pane, then re-check it.
6. Import the calendars you exported in step 2, if any, by double-clicking the files.
7. If the issue is resolved, close the Finder window you opened in step 4. Delete the exported files and the Calendars folder on the Desktop.