$day = (int) date('N');
$hour = (int) date('H');
if($date < 1 or $date > 5 or $hour < 19 or $hour > 20)
echo "<p>This page is only viewable Mon.-Fri. from 7pm to 9pm Eastern US time</p>";
exit; // alternatively, don't exit but instead use an else block for the rest of the page
// rest of page here...
Of course, you'll want to flesh it out to make sure the HTML is complete and valid, but this should give you something to work with.