I found my answer, this is my result:

// Get my 2 array's and put them in a variable 
$januari = (array)$_GET["jan"];
$januaridat = (array)$_GET["date"];

//Get the number of array keys so i can loop with that number
$countkeytot = count((array_keys($januari)));

//Start the loop to check wich ones are selected
for ($i=0; $i<$countkeytot;$i++){
    if ($januari[$i] == 'on'){
	    echo "januari is selected<br>";
            print_r($januaridat[$i])
    }
    else{
	    echo 'januari is NOT selected<br>';
    }
}

This code works fine so far and the output is right, the only thing i still need is when januari is selected i can display the dates that correspond with the checkboxes. Herebeneath my output so far:

Array ( [0] => off [1] => off [2] => off [3] => on [4] => on [5] => on ) 
Array ( [0] => 02-02-2002 [1] => 01-01-2001 [2] => 03-03-2003 [3] => 05-05-2005 [4] => 04-04-2004 [5] => 06-06-2006 ) 
januari is NOT selected
januari is NOT selected
januari is NOT selected
januari is selected
05-05-2005
januari is selected 
04-04-2004
januari is selected 
06-06-2006