On the subject of traffic spikes, while ELBs are impressively performant, there might be occasions where you will need to pre-warm them.
As you probably already know, an ELB is a collection of EC2 instances managed by AWS, running proprietary load balancing software.
An algorithm ensures that the number of ELB EC2 instances grows or shrinks in response to the traffic pattern of your application. This process of adaptive scaling is done based on averaged traffic measurements taken over time and as such is not very rapid.
To ensure that this feature does not become a problem, AWS allows you to request an ELB to be pre-warmed, that is to say, scaled-up ahead of time.
If you are on the premium support plan, you could probably wait until a few hours prior to the event; otherwise, you should contact the support team sooner to account for the extra response time.
You will be asked a series of questions relating to the expected requests per second, average payload size, event...