PHP Day of the Week used as conditional link - php

Question

Using this question I was able to build forward/backward functionality based on the current day of the week using a href links. I also wanted to be able to have a a href link for a specific day so you'll notice I added the $link_7 for example and built a Sunday a href. Here is where the crazyness comes in. You'll Notice $link_7 is Specified and is for Sunday but I have to make the actual href a 2 for Sunday. Today is Friday so I think my routine is doing a +2 and throwing off the value. What is the best way to circumvent this when going direct to a Day of the week?
if (array_key_exists('day', $_GET)) {
$add_day = $_GET['day'];
} else {
$add_day = 0;
}
$yesterday = $add_day - 1;
$tomorrow = $add_day + 1;
$link_yesterday = $_SERVER['PHP_SELF'] . '?day=' . $yesterday ;
$link_tomorrow = $_SERVER['PHP_SELF'] . '?day=' . $tomorrow ;
$link_7 = $_SERVER['PHP_SELF'] . '?day=7' ;
$link_1 = $_SERVER['PHP_SELF'] . '?day=1' ;
$link_2 = $_SERVER['PHP_SELF'] . '?day=2' ;
$link_3 = $_SERVER['PHP_SELF'] . '?day=3' ;
$link_4 = $_SERVER['PHP_SELF'] . '?day=4' ;
$link_5 = $_SERVER['PHP_SELF'] . '?day=5' ;
$link_6 = $_SERVER['PHP_SELF'] . '?day=6' ;
$display_day = mktime(0, 0, 0, date("m") , date("d") + $add_day, date("Y"));
$today = date('N', $display_day);
<a style="color:white" href="<?php echo $link_yesterday;?>">See Yesterday</a>
<a style="color:white" href="<?php echo $link_tomorrow;?>">See Tomorrow</a> |
<a style="color:white" href="<?php echo $link_2;?>">Sunday</a>
<a style="color:white" href="<?php echo $link_3;?>">Monday</a>
<a style="color:white" href="<?php echo $link_4;?>">Tuesday</a>
<a style="color:white" href="<?php echo $link_5;?>">Wednesday</a>
<a style="color:white" href="<?php echo $link_6;?>">Thursday</a>
<a style="color:white" href="<?php echo $link_7;?>">Friday</a>
<a style="color:white" href="<?php echo $link_1;?>">Saturday</a>
if( $today == 7) { ?>
<th class="WADAResultsTableHeader">Sunday</th>
<?php } elseif( $today == 1 ) { ?>
<th class="WADAResultsTableHeader">Monday</th>
<?php } elseif( $today == 2 ) { ?>
<th class="WADAResultsTableHeader">Tuesday</th>
<?php } elseif( $today == 3 ) { ?>
<th class="WADAResultsTableHeader">Wednesday</th>
<?php } elseif( $today == 4 ) { ?>
<th class="WADAResultsTableHeader">Thursday</th>
<?php } elseif( $today == 5 ) { ?>
<th class="WADAResultsTableHeader">Friday</th>
<?php } elseif( $today == 6 ) { ?>
<th class="WADAResultsTableHeader">Saturday</th>
<?php } ?>