diary-header-line-flag

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

t

Documentation:
Non-nil means `diary-simple-display' will show a header line.
The format of the header is specified by `diary-header-line-format'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.