There are three possible banners:
motd - "Message of the day" (but usually updated far less frequently) - appears at first connection.
login - appears after motd
exec - appears after authentication.
Syntax to change, from global configuration mode:
banner motd % This is the message of the day. Today is Tuesday. %
banner login % Unauthorized access is prohibited. Go away. %
banner exec % You have successfully logged in. Don't forget to save your changes. %
Note that the % is a delimiter. It can be any character that isn't part of the message itself. Type it once before the message and once after. If your message contains a % then use something else.
You can insert returns within the message to make it more readable. For example:
banner login %
Unauthorized access is prohibited. Go away.