And here's the patch, sorry..

[Ingo Lohmar <i.lohmar@gmail.com>]

 ChangeLog                 |  5 +++++
 lisp/calendar/calendar.el | 12 +++++++++++-

diff --git a/ChangeLog b/ChangeLog
index 41bff0e..a36e180 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2015-02-21  Ingo Lohmar  <i.lohmar@gmail.com>
+
+	* lisp/calendar/calendar.el (calendar-generate-month): New
+	variable calendar-weekend-days to fontify day headers correctly.
+
 2015-02-21  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Merge from gnulib
diff --git a/lisp/calendar/calendar.el b/lisp/calendar/calendar.el
index e42a010..5e8e9f1 100644
--- a/lisp/calendar/calendar.el
+++ b/lisp/calendar/calendar.el
@@ -190,6 +190,16 @@ movement commands will not work correctly."
          (calendar-redraw))
   :group 'calendar)
 
+(defcustom calendar-weekend-days '(0 6)
+  "Days of the week considered weekend days.
+0 means Sunday, 1 means Monday, and so on.
+
+Determines which day headers are fontified with
+`calendar-weekend-header'."
+  :type '(repeat integer)
+  :version "24.5"
+  :group 'calendar)
+
 (defcustom calendar-view-diary-initially-flag nil
   "Non-nil means display current date's diary entries on entry to calendar.
 The diary is displayed in another window when the calendar is first displayed,
@@ -1453,7 +1463,7 @@ line."
      (insert
       (truncate-string-to-width
        (propertize (calendar-day-name j 'header t)
-                   'font-lock-face (if (memq j '(0 6))
+                   'font-lock-face (if (memq j calendar-weekend-days)
                                        'calendar-weekend-header
                                      'calendar-weekday-header))
        calendar-day-header-width nil ?\s)