Allow customization of calendar month header text

* lisp/calendar/calendar.el (calendar-american-month-header)
(calendar-european-month-header, calendar-iso-month-header)
(calendar-month-header): New options.
(calendar-set-date-style): Set calendar-month-header.  Redraw calendar.
(calendar-generate-month): Use calendar-month-header.

Fixes: debbugs:9510

1 files changed, 67 insertions, 4 deletions

diff --git a/lisp/calendar/calendar.el b/lisp/calendar/calendar.el
index cdbf8d7aa86..93ef440541e 100644
--- a/lisp/calendar/calendar.el
+++ b/lisp/calendar/calendar.el
@@ -921,6 +921,64 @@ styles."
                                    calendar-american-date-display-form)
   :group 'calendar)
 
+(defcustom calendar-american-month-header
+  '(propertize (format "%s %d" (calendar-month-name month) year)
+               'font-lock-face 'font-lock-function-name-face)
+  "Default format for calendar month headings with the American date style.
+Normally you should not customize this, but `calender-month-header'."
+  :group 'calendar
+  :risky t
+  :type 'sexp
+  :version "24.3")
+
+(defcustom calendar-european-month-header
+  '(propertize (format "%s %d" (calendar-month-name month) year)
+               'font-lock-face 'font-lock-function-name-face)
+  "Default format for calendar month headings with the European date style.
+Normally you should not customize this, but `calender-month-header'."
+  :group 'calendar
+  :risky t
+  :type 'sexp
+  :version "24.3")
+
+(defcustom calendar-iso-month-header
+  '(propertize (format "%d %s" year (calendar-month-name month))
+               'font-lock-face 'font-lock-function-name-face)
+  "Default format for calendar month headings with the ISO date style.
+Normally you should not customize this, but `calender-month-header'."
+  :group 'calendar
+  :risky t
+  :type 'sexp
+  :version "24.3")
+
+(defcustom calendar-month-header
+  (cond ((eq calendar-date-style 'iso)
+         calendar-iso-month-header)
+        ((eq calendar-date-style 'european)
+         calendar-european-month-header)
+        (t calendar-american-month-header))
+  "Expression to evaluate to return the calendar month headings.
+When this expression is evaluated, the variables MONTH and YEAR are
+integers appropriate to the relevant month.  The result is padded
+to the width of `calendar-month-digit-width'.
+
+For examples of three common styles, see `calendar-american-month-header',
+`calendar-european-month-header', and `calendar-iso-month-header'.
+
+Changing this variable without using customize has no effect on
+pre-existing calendar windows."
+  :group 'calendar
+  :initialize 'custom-initialize-default
+  :risky t
+  :set (lambda (sym val)
+         (set sym val)
+         (calendar-redraw))
+  :set-after '(calendar-date-style calendar-american-month-header
+                                   calendar-european-month-header
+                                   calendar-iso-month-header)
+  :type 'sexp
+  :version "24.3")
+
 (defun calendar-set-date-style (style)
   "Set the style of calendar and diary dates to STYLE (a symbol).
 The valid styles are described in the documentation of `calendar-date-style'."
@@ -934,8 +992,11 @@ The valid styles are described in the documentation of `calendar-date-style'."
         calendar-date-display-form
         (symbol-value (intern-soft
                        (format "calendar-%s-date-display-form" style)))
+        calendar-month-header
+        (symbol-value (intern-soft (format "calendar-%s-month-header" style)))
         diary-date-forms
         (symbol-value (intern-soft (format "diary-%s-date-forms" style))))
+  (calendar-redraw)
   (calendar-update-mode-line))
 
 (defun european-calendar ()
@@ -1463,9 +1524,8 @@ line."
    (goto-char (point-min))
    (calendar-move-to-column indent)
    (insert
-    (calendar-string-spread
-     (list (format "%s %d" (calendar-month-name month) year))
-     ?\s calendar-month-digit-width))
+    (calendar-string-spread (list calendar-month-header)
+                            ?\s calendar-month-digit-width))
    (calendar-ensure-newline)
    (calendar-insert-at-column indent calendar-intermonth-header trunc)
    ;; Use the first two characters of each day to head the columns.
@@ -2222,9 +2282,12 @@ Negative years are interpreted as years BC; -1 being 1 BC, and so on."
      (- mon2 mon1)))
 
 (defvar calendar-font-lock-keywords
+  ;; Month and year.  Not really needed now that calendar-month-header
+  ;; contains propertize, and not correct for non-american forms
+  ;; of that variable.
   `((,(concat (regexp-opt (mapcar 'identity calendar-month-name-array) t)
               " -?[0-9]+")
-     . font-lock-function-name-face) ; month and year
+     . font-lock-function-name-face)
     (,(regexp-opt
        (list (substring (aref calendar-day-name-array 6)
                         0 calendar-day-header-width)