diary-header-line-format is a variable defined in `diary-lib.el'.
Its value is

(:eval (calendar-string-spread (list (if diary-selective-display "Some text is hidden - press \"C-c C-s\" before edit/copy" "Diary")) 32 (window-width)))

Format of the header line displayed by `diary-simple-display'.
Only used if `diary-header-line-flag' is non-nil.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.3 of Emacs.