I need to automatically assign an EIP

Normally, you can use the Dashboard to manage your Elastic IPs and configure instances accordingly.  However, there may be situations where you'll want to automatically associate an Elastic IP to a new instance when it is launched.  

For example, you might have a server that is only available to visitors one day a week.  You can write a script that will automatically attach the same Elastic IP to an instance when it is launched on the scheduled day.