org-calendar-to-agenda-key

org-calendar-to-agenda-key is a variable defined in `org.el'.
Its value is

[99]

Documentation:
The key to be installed in `calendar-mode-map' for switching to the agenda.
The command `org-calendar-goto-agenda' will be bound to this key. The
default is the character `c' because then `c' can be used to switch back and
forth between agenda and calendar.

You can customize this variable.