motd - message of the day
The contents of /etc/motd are displayed by
pam_motd(8) after a successful login but just before it
executes the login shell.
The abbreviation "motd" stands for "message of the day", and this file has been traditionally used for exactly that (it requires much less disk space than mail to all users).
On Debian GNU/Linux, dynamic content configured at
/etc/pam.d/login is also displayed by pam_exec
/etc/motd