Q: Need to set a custom page as Default landing page after SSO authentication I have SAML 2.0 based SSO authenitcation set in our Salesforce Org. What has to be done so that after successful authentication of the users, they should be landed on a custom page ? 
Setting up the Default Landing page under apps will only work when the user enters after giving the Org credentials. 

A: Is this what you are asking for - Using the StartURL Parameter?

To direct your users to a specific location after authenticating, you
  need to specify a URL with the startURL request parameter. This URL
  must be a relative URL; passing an absolute URL results in an error.
  If you don't add startURL, the user is sent to either /home/home.jsp
  (for a portal or standard application) or to the default sites page
  (for a site) after authentication completes.


A: If you are using SAML 2.0, you can also use the RelayState parameter to control where users get redirected after a successful
login.
Example:
https://mydomain.my.salesforce.com/idp/login?app=00000000000&RelayState=/customers/s/case/Case/00BD65400078CmqMAE

