How do I enable the Debug menu in Calendar?
A menu is most desirable, so this is not a proper answer, but here goes …

Without the debug menu

It's possible to do at least some of the things that were possible with the menu.

Top sekret

Some of the things that were in the Top Sekret submenu of the Debug menu in iCal …

Days in week view

5 or 7 days: Use the General pane of Calendar preferences. 

14, 21 or 28 days:

defaults write com.apple.iCal "n days of week" 14

defaults write com.apple.iCal "n days of week" 21

defaults write com.apple.iCal "n days of week" 28


The change will be effective when you next switch to week view. 

New calendar window

⌘+L is not effective. I'll investigate …