That would be midnight EASTERN time. Whatever time that is in your time zone. And you can't already be logged into your reservation at midnight. You must log in after midnight, as the system seems to only recognize when you logged in as the time it is.
But do make sure you have already logged in at some time previous. You don't want to be setting up your account at midnight when you could be reserving stuff. Also, don't be alarmed/surprised if you can't get in exactly at midnight. Lots of people are doing the same thing. Just keep trying and you'll get in soon.
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