message-citation-line-format

message-citation-line-format is a variable defined in `message.el'.
Its value is

"On %a, %b %d %Y, %N wrote:
"

Documentation:
Format of the "Whomever writes:" line.

The string is formatted using `format-spec'. The following constructs
are replaced:

%f The full From, e.g. "John Doe ".
%n The mail address, e.g. "john.doe@example.invalid".
%N The real name if present, e.g.: "John Doe", else fall
back to the mail address.
%F The first name if present, e.g.: "John".
%L The last name if present, e.g.: "Doe".
%Z, %z The time zone in the numeric form, e.g.:"+0000".

All other format specifiers are passed to `format-time-string'
which is called using the date from the article your replying to, but
the date in the formatted string will be expressed in the author's
time zone as much as possible.
Extracting the first (%F) and last name (%L) is done heuristically,
so you should always check it yourself.

Please also read the note in the documentation of
`message-citation-line-function'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.