Sorry for taking so long to get back to you. My solution would be to create a script "/bin/welcome.sh" with the following
# Print some blank lines and make it beep because we can
echo -e \\n\\n\\n\\a
# Print the message to the terminal
/bin/date +" Welcome $USER. Today is %A %D, and it is now %R";
# Uncomment sleep to make the window auto close after N seconds OR
# Uncomment read to make the user to close the window or press <-'
Then ussing the session manager you can create an entry that runs this command when a the user logs in:
xterm -T "Welcome..." -geometry 75x10+250+250 -bg yellow -fg navy -e /bin/welcome.sh
This should open a nice xterm window with the message in it. I have left commens in the script to give you a choice of whether the windows disappears after a chosen time or whether the user should cancel it.