需要在第二天显示(脚本显示当天)

<body>
<?php
date_default_timezone_set('Pacific/Fiji');
$day = date('l', mktime());
$day = strtolower($day).".html";

/*
$path = $_SERVER['SCRIPT_FILENAME'];
$trace = explode("/", $path);
$count = count ($trace) - 1;
$trace[$count] = $day;
$path = implode("/", $trace);
echo $path;
*/

if (file_exists($day)) 
{
    include($day); 
}
?>
</body>

Simple way to get next day is to use strtotime:

echo date('l', strtotime(date("Y-m-d") .' +1 day'));

You can skip date("Y-m-d"), i put in there to show you, that you can get +1 day from any date you want

Did you try anything?

$date = new DateTime('2015-10-27 12:00:00'); // pick a date (today)

echo $date->format('Y-m-d H:i:s'); // show it

$date->modify('+1 day'); // add + 1 day

echo $date->format('Y-m-d H:i:s'); // shows tomorrow
<body>
<?php
date_default_timezone_set('Pacific/Fiji');
$day = date('l', strtotime('+1 day'));
/*
$path = $_SERVER['SCRIPT_FILENAME'];
$trace = explode("/", $path);
$count = count ($trace) - 1;
$trace[$count] = $day;
$path = implode("/", $trace);
echo $path;
*/

if (file_exists($day)) 
{
    include($day); 
}
?>
</body>

Updated changes for your code to show next day of the week.

<body>    
<?php    
date_default_timezone_set('Pacific/Fiji');    
$day = date('l', strtotime(date($day) .' +1 day'));    
$day = strtolower($day).".html";
/*    
$path = $_SERVER['SCRIPT_FILENAME'];    
$trace = explode("/", $path);    
$count = count ($trace) - 1;    
$trace[$count] = $day;    
$path = implode("/", $trace);    
echo $path;    
*/

if (file_exists($day))     
{    
    include($day);     
}    
?></body>