The message of the day file, /etc/motd, is intended to provide the greeting displayed whenever a user logs in. By default, this file is empty. /etc/motd is an ordinary text file, so the system administrator can change the message by editing the file with a text editor.
In general, you should limit the size of the /etc/motd file to include no more than a single screen of information. You can modify this file to include messages such as a reminder to clean up directories, to preserve disk space, a notice of the next periodic backup, a description of the latest system upgrade, or information about upcoming scheduled system down times.