Earlier quoted context omitted.
Because it isn't legally "published" until the next business day and Monday is a federal holiday. Federal law says rulemaking isn't valid until 180 days have passed so it kicks in July 17. All of these rules are to keep federal agencies from changing the rules out from under people, and to give Congress/Courts time to act.
Interesting. Thanks for the background and links. So this is how federal agencies that report to the executive branch legislate. I had always wondered what the actual process was.
They don't "legislate" they "regulate".