calendar-date-echo-text

calendar-date-echo-text is a variable defined in `calendar.el'.
Its value is

"mouse-2: general menu
mouse-3: menu for this date"

• This variable may be risky if used as a file-local variable.

Documentation:
String displayed when the cursor is over a date in the calendar.
Can be either a fixed string, or a lisp expression that returns one.
When this expression is evaluated, DAY, MONTH, and YEAR are
integers appropriate to the relevant date. For example, to
display the ISO date:

(setq calendar-date-echo-text '(format "ISO date: %s"
(calendar-iso-date-string
(list month day year))))
Changing this variable without using customize has no effect on
pre-existing calendar windows.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.