How do you put up a maintenance page for AWS when your instances are behind an ELB?

The simplest way on AWS is to use Route 53, their DNS service.

You can use the feature of Weighted Round Robin.

"You can use WRR to bring servers into production, perform A/B testing, or balance your traffic across regions or data centers of varying sizes."

More information in AWS documentations on this feature

EDIT: Route 53 recently added a new feature that allows DNS Failover to S3. Check their documentation for more details: http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/dns-failover.html

I realise this is an old question but after facing the same problem today (December 2018), it looks like there is another way to solve this problem.

Earlier this year, AWS introduced support for redirects and fixed responses to Application Load Balancers. In a nutshell:

• Locate your ELB in the console.
• View the rules for the appropriate listener.
• Add a fixed 503 response rule for your application's host name.
• Optionally provide a text/plain or text/html response (i.e. your maintenance page HTML).
• Save changes.

Once the rule propagates to the ELB (took ~30 seconds for me), when you try to visit your host in your browser, you'll be shown the 503 maintenance page.

When your deployment completes, simply remove the rule you added.

Route53 is not a good solution for this problem. It takes a significant amount of time for DNS entries to expire before the maintenance page shows up (and then it takes that same amount of time before they update after maintenance is complete). I realize that Lambda and CodeDeploy triggers did not exist at the time this question was asked, but I wanted to let others know that Lambda can be used to create a relatively clean solution for this, which I have detailed in a blog post: http://blog.ajhodges.com/2016/04/aws-lambda-setting-temporary.html

The jist of the solution is to subscribe a Lambda function to CodeDeploy events, which replaces your ASG with a micro instance serving a static page in your load balancer during deployments.