How to make a sign

There it is, at every entrance to the terminal at LaGuardia, one of the busiest airports in the world:

TERMINAL CLOSED
FOR MAINTENANCE

between 12:00 a.m.
and 4:00 a.m.
until further notice.

Ticketed Passengers &
Employees ONLY
will have access
to Terminal.

A few questions on our way to fixing this:

Who is it for?

What impact will it have on everyone else?

To the sign maker: Are you angry? Frustrated? Trying to teach people a lesson?

What does it sound like when you read it aloud?

and... is it clear?

When I look at this sign, I wonder why it needs to say, "until further notice." After all, aren't all rules in place until further notice?

And why say 12:00 a.m. when midnight is so much more clear?

Do we really need to alert employees to this rule every day?

Mostly, though, the headline is confusing (every person reading this sign for the first time is sure the terminal is closed right this minute, until they read the next line).

Perhaps, then, it might be a better sign if it said:

Hi. To keep this terminal clean, it's closed
to visitors from midnight until 4 a.m. every night.
Ticketed passengers are always welcome.

Thank you.