calendar-mode-line-format

calendar-mode-line-format is a variable defined in `calendar.el'.
Its value is shown below.

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

Documentation:
If non-nil, the mode line of the calendar buffer.
This is a list of items that evaluate to strings. The elements
are evaluated and concatenated, evenly separated by blanks.
During evaluation, the variable `date' is available as the date
nearest the cursor (or today's date if that fails). To update
the mode-line as the cursor moves, add
`calendar-update-mode-line' to `calendar-move-hook'.

If nil, do not modify the mode line at all.

Here is an example that has the Hebrew date, the day number/days
remaining in the year, and the ISO week/year numbers:

(list
""
'(calendar-hebrew-date-string date)
'(let* ((year (calendar-extract-year date))
(d (calendar-day-number date))
(days-remaining
(- (calendar-day-number (list 12 31 year)) d)))
(format "%d/%d" d days-remaining))
'(let* ((d (calendar-absolute-from-gregorian date))
(iso-date (calendar-iso-from-absolute d)))
(format "ISO week %d of %d"
(calendar-extract-month iso-date)
(calendar-extract-year iso-date)))
""))

You can customize this variable.

Value:

(#("<" 0 1
   (keymap
    (keymap
     (mode-line keymap
		(mouse-1 . calendar-scroll-right)))
    mouse-face mode-line-highlight help-echo "mouse-1: previous month"))
 "Calendar"
 #("? info / o other / . today" 0 6
   (help-echo "mouse-1: read Info on Calendar" mouse-face mode-line-highlight keymap
	      (keymap
	       (mode-line keymap
			  (mouse-1 . calendar-goto-info-node))))
   9 16
   (help-echo "mouse-1: choose another month" mouse-face mode-line-highlight keymap
	      (keymap
	       (mode-line keymap
			  (mouse-1 . calendar-other-month))))
   19 26
   (help-echo "mouse-1: go to today's date" mouse-face mode-line-highlight keymap
	      (keymap
	       (mode-line keymap
			  (mouse-1 . calendar-goto-today)))))
 (calendar-date-string
  (calendar-current-date)
  t)
 #(">" 0 1
   (keymap
    (keymap
     (mode-line keymap
		(mouse-1 . calendar-scroll-left)))
    mouse-face mode-line-highlight help-echo "mouse-1: next month")))