<?php 
    $dates = array(
        '2012-01-16',
        '2012-01-26',
        '2012-01-27',
        '2012-02-02',
        '2012-03-21',
        '2012-03-22'
    );

    $dateArray=array();
    foreach ($dates as $date){
        $d = new DateTime($date);
        $monthName = $d->format('F');
        $dateArray[$monthName][] = $date;
    }

   var_dump($dateArray);

?>

array(3) {
  ["January"]=>
  array(3) {
    [0]=>
    string(10) "2012-01-16"
    [1]=>
    string(10) "2012-01-26"
    [2]=>
    string(10) "2012-01-27"
  }
  ["February"]=>
  array(1) {
    [0]=>
    string(10) "2012-02-02"
  }
  ["March"]=>
  array(2) {
    [0]=>
    string(10) "2012-03-21"
    [1]=>
    string(10) "2012-03-22"
  }
}